Add usage doc for node usage

Author: tmcw

docs/USAGE_NODE.md

@@ -0,0 +1,30 @@
+# Using documentation.js as a node library
+
+You might want to do this if you're
+
+* building an integration, like our gulp or grunt integrations
+* using documentation.js's AST parsing or some other component
+* mad science
+
+Basic concepts:
+
+* documentation.js takes an array of entry points, which can be filenames
+  or objects with `source` and `file` members
+* generating documentation is a two-step process: parsing, in the
+  documentation.build and documentation.buildSync methods, and generating
+  output in documentation.formats.md, json, or html.
+
+### Example
+
+```js
+var documentation = require('./');
+
+var docs = documentation.buildSync([{
+  source: '/** hi this is a doc\n@name myDoc */',
+  file: 'direct.js'
+}]);
+
+documentation.formats.md(docs, {}, function(err, res) {
+  console.log(res);
+});
+```