Fix incorrect function call in UPGRADING.md (googleapis#996)

adds service account, OAuth, general usage, and proxying to README (g…

…oogleapis#972)

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

fixes branch alias for dev v1-master

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