feat(readme): Add link to Promise documentation

This is an excuse to generate a new package. Lately I'm getting shasum errors when installing
packages from npm that I uploaded. The easies workaround is generate a new package

Author: jcollado

README.md

@@ -21,7 +21,7 @@ npm install --save promisify-function
 
 ## Usage
 
-Pass a callback style function and get a function the returns a promise as result:
+Pass a callback style function and get a function the returns a [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) as result:
 
 ```node
 const fs = require('fs')