Add pip instruction [skip ci]

Author: stefanradev93

INSTALL.rst

@@ -34,6 +34,15 @@ and activate via
 
    conda activate bf
 
+Install via pip
+------------------
+
+Install BayesFlow from PyPI via
+
+.. code-block:: bash
+
+   pip install bayesflow
+
 Install from GitHub
 -------------------