📝 CONTRIBUTING: update "Pull Request" chapter

Reflects Combodo/iTop#371

CONTRIBUTING.md

@@ -90,8 +90,10 @@ Our tests are located in the `test/` directory, containing a PHPUnit config file
 
 When your code is working, please:
 
-* stash as much as possible your commits,
-* rebase your branch on our repo last commit,
-* create a pull request.
-
-Detailed procedure to work on fork and create PR is available [in GitHub help pages](https://help.github.com/articles/creating-a-pull-request-from-a-fork/).
+* Squash as much as possible your commits,
+* Rebase your branch on our repo last commit,
+* Create a pull request. _Detailed procedure to work on fork and create PR is available [in GitHub help pages](https://help.github.com/articles/creating-a-pull-request-from-a-fork/)_.
+* Pull request description: mind to add all the information useful to understand why you're suggesting this modification and anything necessary to dive into your work. Especially:
+  - Bugfixes: exact steps to reproduce the bug (given/when/then), description of the bug cause and what solution is implemented 
+  - Enhancements: use cases, implementation details if needed
+* Mind to check the "[Allow edits from maintainers](https://docs.github.com/en/github-ae@latest/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork)" option !