give better instructions for extras

README.md

@@ -15,15 +15,15 @@ E-beam doses, and the ability to output to multiple formats.
 Requirements:
 * python >= 3.5 (written and tested with 3.6)
 * numpy
-* matplotlib (optional, used for visualization functions and text)
-* python-gdsii (optional, used for gdsii i/o)
-* svgwrite (optional, used for svg output)
-* freetype (optional, used for text)
+* matplotlib (optional, used for `visualization` functions and `text`)
+* python-gdsii (optional, used for `gdsii` i/o)
+* svgwrite (optional, used for `svg` output)
+* freetype (optional, used for `text`)
 
 
 Install with pip:
 ```bash
-pip3 install masque
+pip3 install 'masque[visualization,gdsii,svg,text]'
 ```
 
 Alternatively, install from git