README: add a logo. [ci skip]

README.md

 # Cabal [![Hackage version](https://img.shields.io/hackage/v/Cabal.svg?label=Hackage)](https://hackage.haskell.org/package/Cabal) [![Stackage version](https://www.stackage.org/package/Cabal/badge/lts?label=Stackage)](https://www.stackage.org/package/Cabal) [![Build Status](https://secure.travis-ci.org/haskell/cabal.svg?branch=master)](http://travis-ci.org/haskell/cabal) [![Windows build status](https://ci.appveyor.com/api/projects/status/github/haskell/cabal?branch=master&svg=true)](https://ci.appveyor.com/project/23Skidoo/cabal) [![Documentation Status](http://readthedocs.org/projects/cabal/badge/?version=latest)](http://cabal.readthedocs.io/en/latest/?badge=latest)
 
+<img src="https://www.haskell.org/cabal/images/Cabal-light.png" align="right">
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 This Cabal Git repository contains the following packages:
 
  * [Cabal](Cabal/README.md): the Cabal library package ([license](Cabal/LICENSE))