README: Make Vale install instructions less verbose
README: Make Vale install instructions less verbose

- I think most people use Homebrew, and if they don't they can figure
  out installation instructions themselves. The 0.9.0 release is really
  old.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs