Update README.md

README.md

@@ -16,7 +16,14 @@ sudo pip install networkx scipy
 python setup.py build
 python setup.py install
 ```
-
+(4.) If you want to see Frame3dd's native output plots and animations, install gnuplot.
+```
+brew install gnuplot
+```
+or
+```
+sudo apt-get install gnuplot
+```
 
 ### Examples