People routinely had issues with our docker-compose files. Usually,

they hit one or more of the following issues:


They'd try to access Review Board's port 8080 instead of Nginx's port

   80, and not see static media.


They'd use the default hostname of reviews.example.com, and fail to

   access it.


They'd select our Postgres config, which had a bug where the default

   user's password wasn't being set.


This change fixes all these, and adds both inline configuration docs and

a guide at the top of both files. The guide contains step-by-step

instructions for configuring their files (including making note of the

importance of the related files/directories for Nginx and Postgres),

and a list of what they may need to consider for production use.
We have better defaults set:

There's no longer an exposed port 8080, so no confusion there.
The default hostname is now localhost, making this easier to use.
memcached now has an explicit memory setting.

There's also a README.md, helping people get some guidance when

looking at the directory in GitHub.

Tested clean installs of both the MySQL and Postgres configurations,

making sure they were usable without any configuration changes.
Rendered the README in a GitHub-compliant Markdown renderer, checking

for rendering issues.