Corrected paths to DNX repository

[michaldudak]

Renamed KRuntime to DNX and corrected repository paths in CONTRIBUTING.md and README.md.

[ghost]

Hi @michaldudak, I'm your friendly neighborhood Microsoft Open Technologies, Inc. Pull Request Bot (You can call me MSOTBOT). Thanks for your contribution!

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[ghost]

@michaldudak, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, MSOTBOT;

[glennc]

Thanks @michaldudak