Add new documentation

doc/usage/runtime.md

@@ -0,0 +1,56 @@
+---
+layout: docs
+title: Optional Runtime
+description: How to use the optional runtime.
+permalink: /docs/usage/runtime/
+---
+
+## Details
+
+6to5 has a few helper functions that'll be placed at the top of the generated
+code if needed so it's not inlined multiple times throughout that file. This may
+become an issue if you have multiple files, especially when you're sending them
+to the browser. gzip alleviates most of this concern but it's still not ideal.
+
+You can tell 6to5 to not place any declarations at the top of your files and
+instead just point them to a reference contained within the runtime.
+
+## Usage
+
+```js
+$ 6to5 --runtime
+```
+
+```js
+to5.transform('code', { runtime: true });
+```
+
+### Getting the runtime
+
+```sh
+$ 6to5-runtime
+```
+
+or
+
+```js
+require('6to5').runtime();
+```
+
+or from an npm release in `runtime.js` from the 6to5 directory.
+
+### Customising namespace
+
+You can also customise the runtime namespace by passing an optional namespace
+argument:
+
+```sh
+$ 6to5-runtime myCustomNamespace
+```
+
+```js
+require("6to5").runtime('myCustomNamespace');
+```
+
+See [Options - runtime](../options) for documentation on changing the reference in
+generated code.