Add license badge in README (#381)

Author: Ocheretovich

README.md

@@ -7,6 +7,7 @@ techniques][cdschecker] to avoid combinatorial explosion.
 
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+[![License](https://img.shields.io/github/license/tokio-rs/loom)](https://github.com/tokio-rs/loom/blob/master/LICENSE)
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