github: add PR template

Author: dfawley

.github/pull_request_template.md

@@ -0,0 +1,54 @@
+Thank you for your PR.  Please follow the steps in this template to ensure a
+swift review.
+
+1. Read and follow the guidelines for contributing here:
+   https://github.com/grpc/grpc-go/blob/master/CONTRIBUTING.md
+
+   Note: if you are submitting a PR that does not address an open issue with an
+   agreed resolution, it is much more likely your PR will be rejected.
+
+2. Read and follow the guidelines for PR titles and descriptions here:
+   https://google.github.io/eng-practices/review/developer/cl-descriptions.html
+
+   *Particularly* the sections "First Line" and "Body is Informative".
+
+   Note: your PR description will be used as the git commit message in a
+   squash-and-merge if your PR is approved.  We may make changes to this as
+   necessary.
+
+3. PR titles should start with the name of the component being addressed, or the
+   type of change.  Examples: transport, client, server, round_robin, xds,
+   cleanup, deps.
+
+4. Does this PR relate to an open issue?  On the first line, please use the tag
+   `Fixes #<issue>` to ensure the issue is closed when the PR is merged.  Or use
+   `Updates #<issue>` if the PR is related to an open issue, but does not fix
+   it.  Consider filing an issue if one does not already exist.
+
+5. PR descriptions *must* conclude with release notes as follows:
+
+   ```
+   RELEASE NOTES:
+   * <componenet>: <summary>
+   ```
+
+   This need not match the PR title.
+
+   The summary must:
+
+   * be something that gRPC users will understand.
+
+   * clearly explain the feature being added, the issue being fixed, or the
+     behavior being changed, etc.  If fixing a bug, be clear about how the bug
+     can be triggered by an end-user.
+
+   * begin with a capital letter and use complete sentences.
+
+   * be as short as possible to describe the change being made.
+
+   If a PR is not end-user visible -- e.g. a cleanup, testing change, or
+   github-related, use `RELEASE NOTES: n/a`.
+
+6. Self-review your code changes before sending your PR.
+
+7. Delete all of the above before sending your PR.