Add TL;DR section to CONTRIBUTING.md

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Tidied up development environment instructions

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Move building binaries instructions

It's less important than signing your work.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #580 from bfirsh/improve-contributing-instructions

Improve contributing instructions