Fix rendering of the README in the app

* Add a missing line break before a list to make it work
* Fix rendering of code blocks in the response code list by switching to
  indented syntax
* Inline the URL standards link into a sentence
* Capitalise some header names and the word "Markdown"
Mark up header names and HTTP statuses as code

Just to make it consistently clear that these are Computer Things.
Use indented code blocks everywhere

Since commit 912306992084e1a1cf350ce19075a8c962a579fe, we're using
intended syntax for multi-line code blocks (Kramdown doesn't understand
GitHub's fenced code block syntax, so they don't render as <pre> blocks
in the app), so we might as well use them consistently.
Merge pull request #75 from alphagov/readme-tweaks

A miscellany of README fixes