Add build badge

Author: long-long-float

README.md

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 # simple-qsim
 
+[![Rust](https://github.com/long-long-float/simple-qsim/actions/workflows/rust.yml/badge.svg)](https://github.com/long-long-float/simple-qsim/actions/workflows/rust.yml)
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 simple-qsim is a simple quantum circuit simulator written in Rust. It simulates quantum states with quantum gates with simple interface.
 
 ## Features