Merge pull request #357 from mathjax/354

[readme] document Promise API

Author: pkra

README.md

@@ -96,9 +96,11 @@ The `start` method start (and restarts) mathjax-node. This allows reconfiguratio
 
 **Note.** This is done automatically when `typeset` is first called (see below).
 
-### `typeset(options, callback)`
+### `typeset(options[, callback])`
 
-The `typeset` method is the main method of mathjax-node. It expects a configuration object `input` and a `callback`.
+The `typeset` method is the main method of mathjax-node. It expects a configuration object `options` and optionally a callback.
+
+If no `callback` is passed, it will return a Promise.
 
 Once called, `typeset` can be called repeatedly and will optionally store information across calls (see `state` below).
 
@@ -132,9 +134,9 @@ The following are the default input options.
 }
 ```
 
-### `callback(result, options)`
+### `Promise.resolve(result,options)` / `Promise.reject(errors)` / `callback(result, options)`
 
-mathjax-node returns two objects to the `callback`: a `result` object as well as the original input `options`.
+mathjax-node returns two objects to `Promise.resolve` or `callback`: a `result` object and the original input `options`.
 
 The `result` object will contain (at most) the following structure:
 
@@ -165,4 +167,6 @@ The `result` object will contain (at most) the following structure:
 }
 ```
 
-The `options` contains the configuration object passed to `typeset`; this can be useful for passing other data along or for identifying which `typeset()` call is associated with this `callback` call (in case you use the same `callback` function for more than one `typeset()`).
+If the `errors` array is non-empty, the Promise will reject, and be passed the `errors` array.
+
+The `options` contains the configuration object passed to `typeset`; this can be useful for passing other data along or for identifying which `typeset()` call is associated with this (`callback`) call (in case you use the same `callback` function for more than one `typeset()`).