Add PR template (#54)

Author: kpamnany

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!---
+PRs to RelationalAI/julia must be opened to the correct branch (see
+https://github.com/RelationalAI/raicode/blob/master/nix/julia-version.json).
+-->
+## PR Kind
+Select only one of:
+- [ ] This is a backport of <link-to-upstream-PR>
+- [ ] This is a RAI-custom patch for <link-to-upstream-issue>
+- [ ] This is a RAI-custom patch for <link-to-JIRA-issue>
+- [ ] Other: <explanation>