Add activate env

Author: bobleesj

README.rst

@@ -104,6 +104,10 @@ Create a new conda environment ``diffpy.pdffit2_env``: ::
 
         conda create -n diffpy.pdffit2_env python=3.12
 
+Activate the environment: ::
+
+        conda activate diffpy.pdffit2_env
+
 Install pdffit2 using ``pip`` to download and install the latest version from `Python Package Index <https://pypi.python.org>`_: ::
 
         pip install diffpy.pdffit2