Update README.md

Author: ffMathy

README.md

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 # substitute.js
-`@fluffy-spoon/substitute` TypeScript port of [NSubstitute](http://nsubstitute.github.io), which aims to provide a much more fluent mocking opportunity for strong-typed languages.
+[`@fluffy-spoon/substitute`](https://www.npmjs.com/package/@fluffy-spoon/substitute) TypeScript port of [NSubstitute](http://nsubstitute.github.io), which aims to provide a much more fluent mocking opportunity for strong-typed languages.
+
+`npm install @fluffy-spoon/substitute --save-dev`
 
 ## Requirements
 * `TypeScript^3.0.0`