Update README for v2.0 stable release (googleapis#968) 

adds PSR-6 and removes custom caching (googleapis#943) * adds PSR-6 and removes custom caching * Adds callback * documents these changes in the README

Merge pull request googleapis#895 from bshaffer/issue-894 passes in cache to token verifier by default

Merge pull request googleapis#887 from bshaffer/update-google-services Update Google Services from Discovery

Merge pull request googleapis#842 from google/issue-821 removes auth library caching

Merge pull request googleapis#816 from gws/configurable-curl-proxy-wo… …rkaround Make cURL proxy header calculation bug workaround configurable

Merge pull request googleapis#768 from google/add-support-for-guzzle-6 Add support for guzzle 6

force 0.4 for google-auth 

Merge pull request googleapis#695 from bshaffer/v2-rcb-1 v2.0 RCB-1