Add PR template mentioning v1 is feature-frozen

Should prevent efforts like #4244 going to waste in the future.

Author: Lonami

.github/pull_request_template.md

@@ -0,0 +1,5 @@
+<!--
+Thanks for the PR! Please keep in mind that v1 is *feature frozen*.
+New features very likely won't be merged, although fixes can be sent.
+All new development should happen in v2. Thanks!
+-->