.github: add PR template (modelcontextprotocol#75)

The default template is too elaborate. Add a simpler one.

Author: jba

.github/pull_request_template.md

@@ -0,0 +1,12 @@
+### PR Tips
+
+Typically, PRs should consist of a single commit, and so should generally follow
+the [rules for Go commit messages](https://go.dev/wiki/CommitMessage), with the following
+changes and additions:
+
+- Markdown is allowed.
+
+- For a pervasive change, use "all" in the title instead of a package name.
+
+- The PR description should provide context (why this change?) and describe the changes
+  at a high level. Changes that are obvious from the diffs don't need to be mentioned.