Revert "Remove erroneous entry in .gitignore"

This reverts commit 604d0c4.

Author: WeatherGod

.gitignore

@@ -45,4 +45,5 @@ lib/matplotlib/mpl-data/matplotlibrc
 #################################
 doc/examples
 doc/_templates/gallery.html
+doc/users/installing.rst
 doc/_static/matplotlibrc