Add license badge to README.

Author: RoiCorporation

README.md

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+[![License: GPL v3](https://img.shields.io/badge/License-GPL%20v3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
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 # vhdl-practice
 Repository with self-made exercises to improve my VHDL skills. The 
 [NVC tool](https://github.com/nickg/nvc) is used as the compiler and simulator