admin_password documentation

The documentation (README file displayed on the charms store) does not mention, that if you skip setting your own admin_password, then setup will FAIL. I had to trawl through the logfiles to find this out.

Secondly, while you are updating the documentation... since there is currently a bug in juju-gui that makes setting config values through there FAIL... I think it would be beneficial to mention explicitly,

"To set admin_password after initial deployment, you can use this syntax:

juju set gitlab admin_password=your_secure_password_here"

Personaly, I find this much nicer than the current suggestion of creating a config.yaml file.