Add new documentation

doc/usage/api.md

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+---
+layout: docs
+title: API
+description: How to use the Node.js API.
+permalink: /docs/usage/api/
+---
+
+
+```javascript
+var to5 = require('6to5');
+```
+
+## to5.transform
+
+Transforms the passed in `code`.
+
+```
+to5.transform(code, [options])
+```
+
+**Example**
+
+```js
+var result = to5.transform('code();', options);
+result.code;
+result.map;
+result.ast;
+```
+
+## to5.transformFile
+
+Asynchronously transforms the entire contents of a file.
+
+```js
+to5.transformFile(filename, [options], callback)
+```
+
+**Example**
+
+```js
+to5.transformFile('filename.js', options, function (err, result) {
+  result.code;
+});
+```
+
+## to5.transformFileSync
+
+Synchronous version of `to5.transformFile`. Returns the transformed contents of
+the `filename`.
+
+```js
+to5.transformFileSync(filename, [options])
+```
+
+**Example**
+
+```js
+to5.transformFileSync('filename.js', options).code;
+```