readme: add examples

Author: eush77

README.md

@@ -13,6 +13,45 @@ Create mutable index mapping property values or computed keys back to [Unist] no
 [david]: https://david-dm.org/eush77/unist-util-index
 [david-badge]: https://david-dm.org/eush77/unist-util-index.png
 
+## Example
+
+Headings by depth:
+
+```js
+var Index = require('unist-util-index'),
+    remark = require('remark'),
+    toString = require('mdast-util-to-string');
+
+var ast = remark.parse(fs.readFileSync('README.md', 'utf8'));
+var index = Index(ast, 'heading', 'depth');
+
+index.get(1).map(toString)
+//=> [ 'unist-util-index' ]
+
+index.get(2).map(toString)
+//=> [ 'Example', 'API', 'Install', 'License' ]
+```
+
+Definitions by identifier:
+
+```js
+var index = Index(ast, 'definition', 'identifier');
+
+index.get('unist')
+//=> [ { type: 'definition',
+         identifier: 'unist',
+         title: null,
+         link: 'https://github.com/wooorm/unist',
+         position: Position { start: [Object], end: [Object], indent: [] } } ]
+
+index.get('travis')
+//=> [ { type: 'definition',
+         identifier: 'travis',
+         title: null,
+         link: 'https://travis-ci.org/eush77/unist-util-index',
+         position: Position { start: [Object], end: [Object], indent: [] } } ]
+```
+
 ## API
 
 ### `index = Index([ast, [filter]], key)`