Add documentation for internal APIs

[stadelmanma]

Sablon has a couple of internal APIs now that allow for end user extensions of the gem. They should be at least mentioned in a section in the README to let users know there are a couple of routes to customize things that don't require a fork and/or PR.

Ones to document:

• Adding new content types
• Adding new DOM file handlers
• Adding new merge field handlers (pending Implement merge field handler registration #99)