Update README.md

Added link to Google documentation on OAuth2.
Add anchor links
Merge pull request #3 from deborahkim/patch-1

Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Merge pull request #4 from jackmichel/patch-1

New section for self-registering a new Enterprise app