Provide template for pull requests

Signed-off-by: Valeriy Zhirnov <neonailol@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

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+Many thanks for your contribution, we genuinely appreciate it. 
+Make sure that you can say "YES" to each point in this short checklist:
+
+  - You do not have merge commits in PR
+  - You made a small number of changes
+  - You provided the link to related issue from YouTrack
+  - You can describe changes made is PR
+  - You made changes related to only one issue
+  - You are ready to defend your changes on code review
+  - You don't touch what you don't understand
+  - You ran the build locally, and it passed
+
+Thank you for your contribution!