Update contribution guidelines
Update contribution guidelines

Include section on semantic versioning.

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