docs: add pull request template (#106)

Author: malnick

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+# Overview
+A high level description of the contribution, including:
+Who the change affects or is for (stakeholders)?
+What is the change? 
+Why is the change needed?
+How does this change affect the user experience (if at all)?
+
+# Design of Change
+How was this change implemented?
+
+# Related Issues/Pull Requests
+[ ] [Issue #1234](https://github.com/hashicorp/vault/issues/1234)
+[ ] [PR #1234](https://github.com/hashicorp/vault/pr/1234)
+
+# Contributor Checklist
+[ ] Add relevant docs to upstream Vault repository, or sufficient reasoning why docs won’t be added yet
+[My Docs PR Link](link)
+[Example](https://github.com/hashicorp/vault/commit/2715f5cec982aabc7b7a6ae878c547f6f475bba6)
+[ ] Add output for any tests not ran in CI to the PR description (eg, acceptance tests)
+[ ] Backwards compatible