Improve Google CA docs (#280)
Improve Google CA docs (#280)

Add a link to the sign up form, and fix the doc link. Some improvements are still necessary after this commit:
- Clarify which portions of the instructions should be followed.
- Clarify the third step "Use the HMAC and KID ... in your configuration file". In what configuration file? How?
google/sxg-rs,google/sxg-rs,google/sxg-rs,google/sxg-rs,google/sxg-rs,google/sxg-rs