add contribution file

Author: 0xc0170

CONTRIBUTING.md

@@ -0,0 +1,5 @@
+# Contributing to Mbed OS
+
+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).

README.md

@@ -61,3 +61,10 @@ More information on the network-socket API can be found in the [mbed handbook](h
 ## Troubleshooting
 
 If you have problems, you can review the [documentation](https://os.mbed.com/docs/latest/tutorials/debugging.html) for suggestions on what could be wrong and how to fix it.
+
+## License and contributions
+
+The software is provided under Apache-2.0 license. Contributions to this project are accepted under the same license. Please see [contributing.md](CONTRIBUTING.md) for more info.
+
+This project contains code from other projects. The original license text is included in those source files. They must comply with our license guide.
+