[docs] add pypi badge

README.md

@@ -1,6 +1,9 @@
 # Hardware-Tailored Stabilizer Circuits Python library
 
 <p align="center">
+  <a href="https://pypi.org/project/htstabilizer/">
+    <img alt="MIT License" src="https://img.shields.io/pypi/v/htstabilizer"/>
+  </a>
   <a href="https://github.com/Mc-Zen/quill/blob/main/LICENSE">
     <img alt="MIT License" src="https://img.shields.io/badge/license-MIT-blue"/>
   </a>
@@ -11,7 +14,6 @@
 
 
 
-
 This library provides 
 - *hardware-tailored* quantum Clifford circuits for **stabilizer state preparation** or **measurement** as well as 
 - mutually unbiased bases in order to perform more efficient **full-state tomography** on small qubit systems.