Add content to api README

src/babel/api/README.md

@@ -1,3 +1,18 @@
 ## API
 
-This is the API directory.
+In this directory you'll find all the public interfaces to using Babel for both
+node and the browser.
+
+### Node
+
+There are two ways people use Babel within Node, they either are manipulating
+strings of code with `babel.transform` or `babel.parse`, they also might be
+running their code through Babel before execution via `register` or `polyfill`.
+
+### Browser
+
+Usage of Babel in the browser is extremely uncommon and in most cases
+considered A Bad Idea™. However it works by loading `<script>`'s with XHR,
+transforming them and then executing them. These `<script>`'s need to have a
+`type` of "text/ecmascript-6", "text/babel", or "module" ("text/6to5" exists as
+well for legacy reasons).