Add GitHub Pull Request template (#2899)

Author: pivovarit

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!--- remove this section after reading it
+
+Currently, there are two significant branches:
+- `master` (represents a stream of work leading to the release of a new major version)
+- `version/0.x` (continues 0.10.5 with minor updates and bugfixes)
+
+That means effectively two things:
+- you can't do breaking changes in `version/0.x` branch
+- it's a good idea to create another Pull Request for compatible changes to `version/0.x` branch
+-->