doc: Add some contributor guidelines

Document some things that should be helpful to at least new
contributors. Since we don't have a way to show this when people are
creating MRs, also copy over the next to a merge request template so
that creates a dropdown that folks might look at when creating an MR.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/444>

.gitlab/merge_request_templates/Guidelines.md

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+# Contribution Guidelines
+
+## A note for contributors
+
+Thank you for your contribution!
+
+Please make sure you tick the box labelled `Allow commits from members who can
+merge to the target branch`. This allows us to make minor edits ourselves, and
+then automatically rebase and merge your changes.
+
+PulseAudio is currently maintained by three volunteer developers in their free
+time (probably amounting to less than one full time developer), which is not
+really enough, given the project size and scope. For this reason bug reports
+and patch submissions are sometimes handled very slowly.
+
+For non-trivial patches, we meet biweekly on IRC to discuss and prioritise
+outstanding MRs. If you haven't heard from us a few days after you create the
+MR, please take a look at [the patch status
+page](https://www.freedesktop.org/wiki/Software/PulseAudio/PatchStatus/).
+
+If you don't see your MR in that list either, please don't hesitate to drop a
+comment pinging us, and we'll try to at least respond and make sure your
+request is tracked on that list.
+
+## Coding Style
+
+Please take a look at the [coding style
+documentation](https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/Developer/CodingStyle/)
+on our wiki.
+
+## Commit messagse
+
+We follow the standard git commit message format of a summary on the first line
+(<=50 characterss for preference, <=72 characters otherwise), followed by a new
+line, followed by a detailed commit message. An additional line at the end may
+link to an issue being fixed by this MR.
+
+The first line is usually a short description of "what" your commit does, and
+the rest of the message describes the "why", along with any additional
+information that readers might need to understand the rationale for the change.
+If in doubt, more verbose is better than less.
+
+If you need to describe the "how" of the commit, that is usually best
+documented along with the code itself.
+
+Commit messages are prefixed with the subsystem being affected. Your best bet
+to figure out what is appropriate is to look at previous commit messages. An
+example:
+
+```
+sink: Reduce chat sink priority
+
+Some gaming sound cards have custom profiles with analog-game and
+analog-chat mappings that exist simultaneously. The game sink should
+have higher priority than the chat sink, but currently there's no way to
+affect the sink priorities from the profile-set configuration (the
+mapping priority is not propagated to the sink priority).
+
+I first thought about adding the mapping priority to the sink priority,
+but that could mess up the prioritization system in
+pa_device_init_priority(). I ended up checking for the intended roles
+property to reduce the chat sink priority. I also reduced the iec958
+priority so that the chat and iec958 sinks don't end up with the same
+priority.
+
+Fixes: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/issues/818
+```

CONTRIBUTING.md

@@ -0,0 +1,67 @@
+# Contribution Guidelines
+
+## A note for contributors
+
+Thank you for your contribution!
+
+Please make sure you tick the box labelled `Allow commits from members who can
+merge to the target branch`. This allows us to make minor edits ourselves, and
+then automatically rebase and merge your changes.
+
+PulseAudio is currently maintained by three volunteer developers in their free
+time (probably amounting to less than one full time developer), which is not
+really enough, given the project size and scope. For this reason bug reports
+and patch submissions are sometimes handled very slowly.
+
+For non-trivial patches, we meet biweekly on IRC to discuss and prioritise
+outstanding MRs. If you haven't heard from us a few days after you create the
+MR, please take a look at [the patch status
+page](https://www.freedesktop.org/wiki/Software/PulseAudio/PatchStatus/).
+
+If you don't see your MR in that list either, please don't hesitate to drop a
+comment pinging us, and we'll try to at least respond and make sure your
+request is tracked on that list.
+
+## Coding Style
+
+Please take a look at the [coding style
+documentation](https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/Developer/CodingStyle/)
+on our wiki.
+
+## Commit messagse
+
+We follow the standard git commit message format of a summary on the first line
+(<=50 characterss for preference, <=72 characters otherwise), followed by a new
+line, followed by a detailed commit message. An additional line at the end may
+link to an issue being fixed by this MR.
+
+The first line is usually a short description of "what" your commit does, and
+the rest of the message describes the "why", along with any additional
+information that readers might need to understand the rationale for the change.
+If in doubt, more verbose is better than less.
+
+If you need to describe the "how" of the commit, that is usually best
+documented along with the code itself.
+
+Commit messages are prefixed with the subsystem being affected. Your best bet
+to figure out what is appropriate is to look at previous commit messages. An
+example:
+
+```
+sink: Reduce chat sink priority
+
+Some gaming sound cards have custom profiles with analog-game and
+analog-chat mappings that exist simultaneously. The game sink should
+have higher priority than the chat sink, but currently there's no way to
+affect the sink priorities from the profile-set configuration (the
+mapping priority is not propagated to the sink priority).
+
+I first thought about adding the mapping priority to the sink priority,
+but that could mess up the prioritization system in
+pa_device_init_priority(). I ended up checking for the intended roles
+property to reduce the chat sink priority. I also reduced the iec958
+priority so that the chat and iec958 sinks don't end up with the same
+priority.
+
+Fixes: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/issues/818
+```