mention branches in the contributing.md

Author: paul-szczepanek-arm

CONTRIBUTING.md

@@ -3,3 +3,7 @@
 Mbed OS is an open-source, device software platform for the Internet of Things. Contributions are an important part of the platform, and our goal is to make it as simple as possible to become a contributor.
 
 To encourage productive collaboration, as well as robust, consistent and maintainable code, we have a set of guidelines for [contributing to Mbed OS](https://os.mbed.com/docs/mbed-os/latest/contributing/index.html).
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+## Branches
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+`Master` branch is for releases only. Please target the `development` branch for all your PRs.