Mention people joining a new team.

If you're joining a new team, with a new codebase, it's really useful to
be able to get up to speed quickly on what each bit does.
Pedantic grammar fixes.

* Hyphenation on compound adjectives
* Capitalisation of "JSON"
* Correct "try and" to "try to": it's better in more formal writing
Merge pull request #56 from alphagov/readme-guidance-amendments

README guidance amendments