Add a homepage and return the README at /readme

As part of making the app friendlier more Hypermedia-y. It also follows the
guidance in the Service Manual:
https://www.gov.uk/service-manual/making-software/apis.html#document-by-discovery-and-example
Move json_examples to /public for linkability

We want to make these easily available over HTTP so that they can be linked to
from live documentation.
Rewrite public links in README to work over HTTP

The README is written for GitHub, where they work as-is, but we want them to
work when viewing the README in the running app.
Tweak README to better display allowed markup

This also improves it when rendered by the app (as opposed to GitHub).
Merge pull request #53 from alphagov/homepage

Homepage