dev: add PR template

Author: hlissner

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+<!-- 
+
+  MAKE SURE YOUR PR MEETS THIS CRITERIA:
+ 
+  - Your PR is NOT in our do-not-PR list:
+    https://doomemacs.org/d/do-not-pr
+  - Your commit messages conform to our git conventions:
+    https://doomemacs.org/d/git-conventions
+  - Any relevant issues or PRs have been linked to.
+  - What the PR does, and why, is properly explained.
+
+-->
+
+Fixes #0000 <!-- remove if not applicable -->
+
+{{{ Summarize what you've changed HERE and why }}}
+
+