feat(doc): describe how to use in readme

Author: bahmutov

README.md

@@ -8,16 +8,43 @@
 [![semantic-release][semantic-image] ][semantic-url]
 [![js-standard-style][standard-image]][standard-url]
 
+## Install
+
+```
+npm install -S stack-sites
+```
+
+```js
+const stackSites = require('stack-sites')
+function foo () {
+  const sites = stackSites()
+  // sites[0] - stackSites itself
+  // sites[1] - this file
+  // sites[2] - caller to this function
+  const caller = sites[2]
+  console.log('I got called from %s line %d column %d',
+    caller.filename, caller.line, caller.column
+  )
+}
+module.exports = foo
+```
+
+## Why?
+
+V8 stack API does not return accurate source locations if the source file
+has been transpiled on load, see
+[Accurate call sites](https://glebbahmutov.com/blog/accurate-call-sites/)
+blog post. Exception stack on the other hand can take source maps into
+the account, producing correct line and column information.
+
 ### Small print
 
 Author: Gleb Bahmutov <gleb.bahmutov@gmail.com> © 2017
 
-
 * [@bahmutov](https://twitter.com/bahmutov)
 * [glebbahmutov.com](http://glebbahmutov.com)
 * [blog](http://glebbahmutov.com/blog)
 
-
 License: MIT - do anything with the code, but don't blame me if it does not work.
 
 Support: if you find any problems with this module, email / tweet /

package.json

@@ -67,5 +67,9 @@
     "nsp": "2.6.2",
     "pre-git": "3.12.0",
     "standard": "8.6.0"
+  },
+  "dependencies": {
+    "check-more-types": "2.23.0",
+    "lazy-ass": "1.5.0"
   }
 }