Update PR template with branch info (#1128)

* Update PR template with branch info

* Update with suggestions from Richie

Author: samodell

.github/pull-request-template.md

@@ -1,4 +1,27 @@
-Fixes #(issue-number)
+<!-- General PR guidelines:
+
+New contributors:
+
+If you are new to Git/GitHub and want to make a quick fix to the docs,
+open your PR against the release branch where you found the error, such as
+"release-0.5".
+
+Regular contributors:
+
+Most PRs should be opened against the master branch.
+
+If the change should also be in the most recent numbered release, add the
+corresponding "cherrypick-0.X" label; for example, "cherrypick-0.5", to the
+original PR. Best practice is to open a PR for the cherry-pick yourself after
+your original PR has been merged into the master branch. Once the cherry-pick PR
+has merged, remove the cherry-pick label from the original PR.
+
+For more information on contributing to the Knative Docs, see:
+https://www.knative.dev/contributing/docs-contributing/
+
+ -->
+
+Fixes #issue-number
 
 ## Proposed Changes