update the "Contributing Policy"

Author: michael-damatov

CONTRIBUTING.md

@@ -5,4 +5,6 @@
 - For typos and spelling mistakes feel free to provide a fix and they will be merged as long they comply with the owner's style guide.
 - For code changes always open an issue first. This will allow to determine whether or not the change should take place. The issue should be used to discuss the change.
 
-- If the pull request is accepted the project collaborators preserve the right to fix the code style before merging the changes
+- If the pull request is accepted the project collaborators preserve the right to fix the code style before merging the changes.
+
+- Pull requests must target the `develop` branch. Pull requests to the `master` branch will be declined.