Update README
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

TFOH/govuk_elements,gavboulton/govuk_elements,TFOH/govuk_elements,joelanman/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,joelanman/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements