Update README.md

Author: ffMathy

README.md

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 # substitute.js
 [`@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.
 
+## Installing
 `npm install @fluffy-spoon/substitute --save-dev`
 
 ## Requirements