Update README.md

README.md

 This Python script converts a TOUGH2 result file to VTK format for visualization with Paraview
+# Install Conda on Mac OS
+* go to https://docs.conda.io/en/latest/miniconda.html
+* download 64-bit (.pkg installer) for Python 3.7 for MAC OS
+* install with .pkg
+* open a terminal
+
+```
+$ conda --version
+conda 4.6.2
+
+$ conda create -n py3 python=3
+
+$ conda activate py3
+```
 
 # Installing required Python packages
 ```