Document the API

I couldn’t find the API in the docs. This is what I found out by reading the source. Let’s save others the pain.

Mark partials as optional

README.md

@@ -111,6 +111,25 @@ var output = Mustache.render("{{title}} spends {{calc}}", view);
 
 In this example, the `Mustache.render` function takes two parameters: 1) the [mustache](http://mustache.github.com/) template and 2) a `view` object that contains the data and code needed to render the template.
 
+## API
+
+Following is an [rtype](https://git.io/rtype) signature of the most commonly used functions.
+
+```js
+Mustache.render(
+  template  : String,
+  view      : Object,
+  partials? : Object,
+) => String
+
+Mustache.parse(
+  template              : String,
+  tags = ['{{', '}}']   : Tags,
+) => String
+
+interface Tags [String, String]
+```
+
 ## Templates
 
 A [mustache](http://mustache.github.com/) template is a string that contains any number of mustache tags. Tags are indicated by the double mustaches that surround them. `{{person}}` is a tag, as is `{{#person}}`. In both examples we refer to `person` as the tag's key. There are several types of tags available in mustache.js, described below.