docs: Add GitHub pull request template

The template informs the user that the GitHub repo is a read-only
mirror and directs the user to the upstream contributor's guide.

Change-Id: Iac759e0b4c18bfd5432a0df3b51a5247eb5cd281
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/205810
Commit-Queue: Kayce Basques <kayce@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: Kayce Basques

.github/pull_request_template.md

@@ -0,0 +1,5 @@
+Thank you for your contribution. Please follow this process to contribute code
+to Pigweed: https://pigweed.dev/docs/get_started/upstream.html
+
+https://github.com/google/pigweed is a read-only mirror of the Pigweed
+repository.