Update README.md

README.md

@@ -17,10 +17,11 @@ python setup.py build
 python setup.py install
 ```
 4. (optional) If you want to see Frame3dd's native output plots and animations, install gnuplot.
+On mac:
 ```
 brew install gnuplot
 ```
-or
+or on Linux
 ```
 sudo apt-get install gnuplot
 ```