Add a PR template with a message about PRs needing to be for master

[aaronfranke]

There are a lot of confused contributors making PRs for the 3.2 branch by mistake, so I think it would be helpful to document the proper procedure by adding some text in a PR template. Here's what I have so far:

<!--
Pull requests must always be made for the "master" branch first.

Relevant fixes are cherry-picked for stable branches as needed.

Do not create a pull request for stable branches unless the change
is already available in the master branch and it cannot be
easily cherry-picked. Alternatively, if the change is only
relevant for that branch (ex: GLES3 fixes for the 3.2 branch)
-->

[Xrayez]

I think "must always" is a bit too strict. I find myself doing 3.2 patches first and then port them to master eventually (or not). I mean that the master-first workflow should not be tied to pull request workflow, but at the end of the day the feature must be first merged to master. I mean, I hope it's fine to target any branch initially for development (as long as the codebase is not too different), unless Akien wants to discourage making PRs that undergo testing in stable branches.

[akien-mga]

Thanks!