Update README

- Explain GOV.UK Elements
- Explain how it can be used
- Encourage contribution via the front end toolkit
- Simplify instructions to view the site locally
Correct `files` to `file` in sentence
Add note about minimum version of node
Add suggested changes to README
Correct typo

- Change his one, to “this” one
- Remove trailing whitespace
Merge pull request #35 from alphagov/update-readme-to-better-explain-elements