PR template and changelog (#1910)

Signed-off-by: Andrey Sobolev <haiodo@gmail.com>

.github/pull_request_template.md

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+# Contribution checklist
+
+## Brief description
+
+## Checklist
+
+[ ] - Does the code work? Check whether function and logic are correct.
+[ ] - Does Changelog.md is updated with changes?
+[ ] - Does the translations are up to date?
+[ ] - Does it well tested?
+[ ] - - Tested for Chrome.
+[ ] - - Tested for Safari.
+[ ] - Go through the changed code looking for typos, TODOs, commented LOCs, debugging pieces of code, etc.
+[ ] - Rebase your branch onto master and upstream branch
+[ ] - Is there any redundant or duplicate code?
+[ ] - Are required links are linked to PR?
+[ ] - Does new code is well documented ?
+
+## Related issues
+
+_TODO_ A list of closed updated issues