Update readme

Mostly formatting and wording changes. The  more substantial edits are

* Added a section for event listeners.
* Removed the part about database migrations, because they don't actually exist on master. See #36 
* Removed the *why use rubot* section. One of the two bullet points was about updating itself from a git repo, but the framework doesn't provide that at all. It's left up to the user to add a gem for git, or whatever vcs, and utilize the reloading feature. This could be included as an example use of reloading, but I'm not going to do that now.