Add a section about TypeScript defs in README

Since TypeScript usage has exploded the last years but this is
package is written in JavaScript, we might at least reference
the external DefinitelyTyped package @types/mustache that has
a somewhat up-to-date set of type definitions for most of the package.

Author: phillipj

README.md

@@ -534,6 +534,11 @@ $ rake yui3
 $ rake qooxdoo
 ```
 
+## TypeScript
+
+Since the source code of this package is written in JavaScript, we follow the [TypeScript publishing docs](https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html) preferred approach
+by having type definitions available via [@types/mustache](https://www.npmjs.com/package/@types/mustache).
+
 ## Testing
 
 In order to run the tests you'll need to install [Node.js](http://nodejs.org/).