Add pip install command and PyPI link to README.md

Author: mmulqueen

README.md

@@ -2,6 +2,14 @@
 
 This is a Python wrapper for the CEL C++ implementation.
 
+## Installation
+
+```
+pip install cel-expr-python
+```
+
+Available on PyPI: https://pypi.org/project/cel-expr-python
+
 ## Usage
 
 ### Importing CEL module