Added a very basic how-to use this image, and a reminder to add a proper example.

buildpack-deps/README-content.md

@@ -6,3 +6,9 @@ includes a large number of "development header" packages needed by various
 things like Ruby Gems, PyPI modules, etc. For example, `buildpack-deps` would
 let you do a `bundle install` in an arbitrary application directory without
 knowing beforehand that `ssl.h` is required to build a dependent module.
+
+# How to use this image
+
+This stack is designed to be the foundation of a language-stack image.
+
+<!-- TODO: add an example -->