Add lib/README.md to mojom package.
Add lib/README.md to mojom package.

This will ensure that mojom/ gets added to packages/ in a consumer environment.

R=iposva@google.com

Review URL: https://codereview.chromium.org/1132513004

chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,collinjackson/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,collinjackson/mojo,jianglu/mojo,collinjackson/mojo,jianglu/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,collinjackson/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo,collinjackson/mojo,afandria/mojo,afandria/mojo,collinjackson/mojo