Add Gerrit User Summit USA 2019 post

The Gerrit User Summit 2019 has now two dates and locations:
- Summer 2019 (Gothenburg - Sweden)
- Autumn 2019 (Sunnyvale - California)

Advertise the new dates to the home page and gives the basic
information needed for the people to sign-up to the event.

Change-Id: I348dd26b495c8ea47aecd923f1c56c6a67ed5af0
1 file changed