README.md: Restore SQLAlchemy docs and links

The explanation for sqlalchemy_index_database was removed in bb69c43f
(update documentation for config_sample.yml changes, 2014-05-14), and
the reference URL for search-endpoint was removed in 4ba12e5d (Updated
documentation for 0.7, new developer (contribute) doc, new changelog
file, 2014-05-20).

Since I added the links in fab832bf (search: Use SQLAlchemy as a
search-index backend, 2014-02-12), the search-endpoint URL has moved
from:

  http://docs.docker.io/en/latest/reference/api/index_api/#get--v1-search

to:

  http://docs.docker.com/reference/api/docker-io_api/#search

But I haven't tracked down the Docker-side commit responsible for that
move.
Merge pull request #472 from wking/readme-restore-sqlalchemy

README.md: Restore SQLAlchemy docs and links