Merge pull request #832 from 0xc0170/fix_license_principle

license: define what acceptence we have for licenses in PR

Author: Amanda Butler

docs/reference/contributing/guidelines/license.md

@@ -106,6 +106,8 @@ If you decide to use a different license for your work, follow the same pattern:
 
 #### Mbed OS principles
 
-Mbed OS uses these same basic principles for its source code and library distributions. So source code we own is distributed under the Apache 2.0 license, and binary blobs are released under the Permissive Binary License. Software parts from third parties that were already licensed under a different license are available under that original license.
+By creating a pull request on GitHub against Mbed OS, you are agreeing to license your contributions under the same license as the original code. Note that Mbed software is released under a permissive license for all new Mbed contributions. Arm will not accept proprietary code or any code under a "copyleft" license. Accepting such code would prevent us from continuing to distribute our code under the permissive license and stall adoption of our code by third parties.
 
-All the source code and binary blobs that end up in Mbed OS are maintained in public GitHub repositories.
+Mbed OS uses these same basic principles for its source code and library distributions. Source code we own is distributed under the Apache 2.0 license, and binary blobs are released under the Permissive Binary License. Software parts from third parties that were already licensed under a different license are available under that original license.
+
+All the source code and binary blobs in Mbed OS are maintained in public GitHub repositories.