SapMachine #178: Update pull_request_template.md

There should be clearer instructions for merging PRs directly in the template text.

.github/pull_request_template.md

@@ -1,4 +1,8 @@
-The description of this pull request goes here.
+Replace this line with a description of this pull request and replace Issue in `fixes #Issue` down below with an issue number. Otherwise PR testing will fail.
 
-fixes #Issue
+When integrating please make sure you:
+- Create a merge commit when merging an OpenJDK upstream PR
+- Use Rebase & Merge when your PR contains only one commit with a commit message of the form `SapMachine #<Issue>: <Description>`
+- Use Squash and merge when there are several commits on the PR and with that update the commit message to `SapMachine #<Issue>: <Description>` and remove commit messages from sub-commits
 
+fixes #Issue