fix #2125: add LICENSE.md file
note about the libs/ directory
remove extra styles in v21/
transform error into warning for NewApi lint check in values/styles*.xml
Merge pull request #2126 from wordpress-mobile/issue/2125-add-license-file

fix #2125: add LICENSE.md file
Merge pull request #2128 from wordpress-mobile/issue/546-stop-build-on-lint-error

Remove extra styles in v21/ and update `lint.xml`
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into feature/calypso-stats