A few more notes of what we expect from a PR (#4108)

* A few more notes of what we expect from a PR

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-authored-by: Anshul Singhvi <[email protected]>

* add section about giving write access

* Apply suggestions from code review

Co-authored-by: Eddie Groshev <[email protected]>

* make a few additions

---------

Co-authored-by: Anshul Singhvi <[email protected]>
Co-authored-by: Eddie Groshev <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,27 @@
+<!--
+* Any PR that is not ready for review should be created as a draft PR.
+* Please don't force push to PRs, it removes the history, creates bad notifications, and we will squash and merge in the end anyways.
+* Feel free to ping for a review when it passes all tests after a few days (@simondanisch, @ffreyer). We can't guarantee a review in a certain time frame, but we can guarantee to forget about PRs after a while.
+* Allowing write access on the PR makes things more convenient, since we can make edits to the PR directly and update it if it gets out of sync with `master` (should be automatic if not disabled).
+* Please understand, that some PRs will take very long to get merged. You can do a few things to optimize the time to get it merged:
+    * The more tests you add, the easier it is to see that your change works without putting the work on us.
+    * The clearer the problem being solved the easier. A PR best only addresses one bug fix or feature.
+    * The more you explain the motivation or describe your feature (best with pictures), the easier it will be for us to priorize the PR.
+    * Changes with more ambigious benefits are best discussed in a github [discussion](https://github.com/MakieOrg/Makie.jl/discussions) before a PR.
+* What deserves a unit test or a reference image test isn't easy to decide, but here are a few pointers:
+   * Makie unit tests are preferable, since they're fast to execute and easy to maintain, so if you can add tests to `Makie/src/test`
+   * For new recipes or any changes that may get rendered differently by the backends, one should add a reference image test to the best fitting file in `Makie\ReferenceTests\src\tests\**` looking like this:
+   ```julia 
+   @reference_test "name of test" begin
+        # code of test
+        ...
+        # make sure the last line is a Figure, FigureAxisPlot or Scene
+    end
+    ``` 
+    Adding a reference image test will let your PR fail with the status "n reference images missing", which a maintainer will need to approve and fix. 
+    Ideally, a comment with a screenshot of the expected output of the reference image test should be added to the PR.
+    We prefer one reference image test with many subplots over multiple reference image tests.
+-->
 # Description
 
 Fixes # (issue)