Add github pull request template.

.github/pull_request_template.md

@@ -0,0 +1,22 @@
+# PLEASE TAKE TIME TO READ THIS #
+
+(Click on `Preview` ☝ to read this more comfortably)
+
+## Before To create a PR
+
+Maybe better to discuss with the team about the change you want to contribute first by [creating a new issue](https://github.com/eclipse/leshan/issues/new).
+
+## Ready to create the PR
+
+Take a look at [Contribution Guide](https://github.com/eclipse/leshan/wiki/How-to-contribute#how-to-contribute-code).
+
+## PR Description
+
+Please include a summary of the change in the PR description.  
+Also, refer corresponding issue if it exists.  
+
+## Your PR is not validated :x:
+
+**DO NOT CREATE A NEW PR**, just fix the current one, See [How to Refine your PR](https://github.com/eclipse/leshan/wiki/How-to-contribute#refine-it).
+
+If you didn't succeed alone, ask for help before to close your PR and create a new one for same issue.