Doc for other supported formats (external helpers)

As Hugo now supports more formats thanks to the new "external helpers"
feature recently introduced, and as requested by some people, I added
some lines in the doc:
* basically confirming it actually exists
* how to use it

docs/content/overview/introduction.md

@@ -85,7 +85,7 @@ links work properly with most hosting companies.
 
 ## What does Hugo do?
 
-In technical terms, Hugo takes a source directory of Markdown files and
+In technical terms, Hugo takes a source directory of files and
 templates and uses these as input to create a complete website.
 
 Hugo boasts the following features:
@@ -114,7 +114,8 @@ Hugo boasts the following features:
 
 ### Content
 
-  * Content written in [Markdown](/content/example/)
+  * Native support for content written in [Markdown](/content/example/)
+    * Support for other languages through _external helpers_, see [supported formats](/content/supported-formats)
   * Support for TOML, YAML and JSON metadata in [frontmatter](/content/front-matter/)
   * Completely [customizable homepage](/layout/homepage/)
   * Support for multiple [content types](/content/types/)