First cut generic Contributing.md file for Travis
First cut generic Contributing.md file for Travis

For https://github.com/travis-ci/travis-ci/issues/940
developerfm/travis-core,GistIcon/travis-core,GistIcon/travis-core,travis-ci/travis-core,final-ci/travis-core,kidaa/travis-core,travis-ci/travis-core,Tiger66639/travis-core,GistIcon/travis-core,final-ci/travis-core,kidaa/travis-core,jantman/travis-core,Tiger66639/travis-core,kidaa/travis-core,final-ci/travis-core,jantman/travis-core,developerfm/travis-core,developerfm/travis-core,Tiger66639/travis-core,travis-ci/travis-core