Rename README to README.md

Switching to the Markdown formatting with some real text. Start by
renaming the thing.
Moved README to markdown

README is now README.md, and is a fair sight better formatted and more
useful.
Formatting fix
Removed extraneous horrizontal rule, as github's markdown autogenerates
them for all headings.

Added a new heading for the usage block, because it looks better.
git rm README

Usually a rename without any text change is picked up, but alas not
here. Forcing deletion of the old README as it's now README.md
Merge branch 'readme-update' into develop

Ugly text README is now a nicely formatted markdown version.