Update README to link to commonly used packages

Moving packaging and publishing documentation to `docs` made it less
clear how to consume `govuk_template`. Add links to the common ways
to consume it.

It's still not super clear, but I don't think the original was either.

Ideally we should re-write the README/docs based on user needs of people
using the packages, based on research

README.md

@@ -1,8 +1,11 @@
-# GovukTemplate
+# GOV.UK Template
 
 This provides a template containing the GOV.UK header and footer, and associated assets.
 
-This is versioned following [Semantic Versioning](http://semver.org).
+The template is built from [`source`](source/) files, and multiple packages are generated to support different languages and frameworks.
+
+Packages are available for [RubyGems](https://rubygems.org/gems/govuk_template), NPM ([mustache](https://npmjs.org/package/govuk_template_mustache), [ejs](https://npmjs.org/package/govuk_template_ejs),  [jinja](https://npmjs.org/package/govuk_template_jinja)), and other [templating languages](docs/packaging.md).
+
 
 ## Previewing the template
 
@@ -26,3 +29,5 @@ The Ruby language (1.9.3+), the build tool [Rake](http://rake.rubyforge.org/) &
 ## Contributing
 
 Please follow the [contribution guidelines](https://github.com/alphagov/govuk_template/blob/master/CONTRIBUTING.md).
+
+This is versioned following [Semantic Versioning](http://semver.org).