Added instructions for installing dev testing package
merged branch tobiassjosten/patch-1 (PR #713)

This PR was squashed before being merged into the master branch (closes #713).

Discussion
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Added instructions for installing dev testing package

As per [discussion about this requirement](https://groups.google.com/d/topic/silex-php/FfgPucE2Bu4/discussion).

Commits
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862cf39 Added instructions for installing dev testing package