update readme with misc improvements

Here are a variety of improvements to the README that I made in the process of configuring the project to use its command-line interface on my client.

The only change I'm uncertain about is the change to the *mysql* line that creates the database. I had to change the username to "root" to make that line work on my Mac, but perhaps that's incorrect for Linux boxes. And perhaps it's wrong for my Mac, too, given that the CLI throws "ER_ACCESS_DENIED_ERROR: Access denied for user 'apk'@'localhost' (using password: YES)" when I try to use the CLI to generate an APK.