added logos

Author: gfursin

README.md

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 Shared programs, benchmarks and kernels for autotuning/crowd-tuning
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+[![logo](https://github.com/ctuning/ck-guide-images/blob/master/logo-powered-by-ck.png)](http://cKnowledge.org)
+[![logo](https://github.com/ctuning/ck-guide-images/blob/master/logo-validated-by-the-community-simple.png)](http://cTuning.org)
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 These are various public programs, benchmarks and kernels used
 in our research on universal and multi-objective autotuning/crowd-tuning
 in the [open Collective Knowledge format](http://cKnowledge.org):