Docs revamp

This is my first attempt to reorganize the docs to surface what Sendak does and how to install it more quickly, and to separate out stuff needed for users of Sendak vs developers of Sendak.

Rough summary:

* Added an intro section, and a minor table of contents.
* Added a usage section to show examples.
* Moved installation instructions into `README.md`, and removed `QUICK_INSTALL.md`.
* Moved "what's in the repo" to a new DEVELOPING.md file.
* I removed mention of rrm and riak-dc, since there was no actual configuration work needed, as well as the `--rrm` environment check.
* Added a little "public domain" blurb at the bottom of the README, for those who don't click through to the license.

I preserved the original prose descriptions of Sendak's purpose and origin, but I do think the README would benefit from a more terse, bullet-y set of use cases.