docs: added Pull Request Guidelines (#838)

Author: mikepsinn

CONTRIBUTING.md

@@ -7,3 +7,6 @@ As this package is related to laravel, we are follows the PSR-2 coding standard
 
 #### StyleCI
 We've integrated StyleCI. So when you submit a PR, it will automatically suggest the fix of styling. Fix it and re-submit the PR if needed.
+
+#### Pull Requests
+Please make all pull requests to the `develop` branch of the project.