update readme

Author: ardean

README.md

@@ -1,6 +1,8 @@
 # jsGBC-core
 
-[![NPM Version][npm-image]][downloads-url] [![NPM Downloads][downloads-image]][downloads-url]
+[![NPM Version][npm-image]][downloads-url]
+[![NPM Downloads][downloads-image]][downloads-url]
+[![License][license-image]][license-url]
 
 **jsGBC Core Emulator**
 
@@ -24,3 +26,5 @@ Z80 implementation borrowed from [GameBoy-Online](https://github.com/taisel/Game
 [downloads-url]: https://npmjs.org/package/jsgbc
 [npm-image]: https://img.shields.io/npm/v/jsgbc.svg
 [npm-url]: https://npmjs.org/package/jsgbc
+[license-image]: https://img.shields.io/npm/l/jsgbc.svg
+[license-url]: LICENSE.md