Add link to documentation in README.md

Since we are using YARD to annotate our code and generate documentation,
and since RubyDoc.info hosts YARD documentation for us, we may as well
link to the documentation over there to help people understand our code
if they are so inclined.

Change-Id: I929f43fb9bbaa0c5fb10fe8c56d165043a56bf92
Scope dump.rdb gitignore to root directory

We only expect this file to show up in the project's root directory.
Scoping this to that directory will allow this file, if it appears
somewhere else, to be seen by git. This probably doesn't actually
matter, but it seems safer and more correct to me.

Change-Id: I5f55395cc98da8cd24c76bea1871813450a8d25c
Add YARD files to .gitignore

We are hosting our documentation on RubyDoc.info[1], so we don't want to
commit it to our repo.

[1]: http://rubydoc.info/github/trotzig/diffux/master/frames

Change-Id: I37b8ce619ee8f389905945cfeddf4ff2a889e2da