Add "Open on run.dlang.io" button

README.md

@@ -1,6 +1,6 @@
 # Optional type for D featuring NotNull
 
-[![Latest version](https://img.shields.io/dub/v/optional.svg)](https://code.dlang.org/packages/optional) [![Build Status](https://travis-ci.org/aliak00/optional.svg?branch=master)](https://travis-ci.org/aliak00/optional) [![codecov](https://codecov.io/gh/aliak00/optional/branch/master/graph/badge.svg)](https://codecov.io/gh/aliak00/optional) [![license](https://img.shields.io/github/license/aliak00/optional.svg)](https://github.com/aliak00/optional/blob/master/LICENSE)
+[![Latest version](https://img.shields.io/dub/v/optional.svg)](https://code.dlang.org/packages/optional) [![Build Status](https://travis-ci.org/aliak00/optional.svg?branch=master)](https://travis-ci.org/aliak00/optional) [![codecov](https://codecov.io/gh/aliak00/optional/branch/master/graph/badge.svg)](https://codecov.io/gh/aliak00/optional) [![license](https://img.shields.io/github/license/aliak00/optional.svg)](https://github.com/aliak00/optional/blob/master/LICENSE) [![Open on run.dlang.io](https://img.shields.io/badge/run.dlang.io-open-blue.svg)](https://run.dlang.io/is/912kVG)
 
 Full API docs available [here](https://aliak00.github.io/optional/optional.html)
 
@@ -319,4 +319,4 @@ f1(sp);
 // c = null; // nope
 // sp = null; // nope
 // c = new C; // nope
-```
+```