Add a tiny bit to Usage Guide index

docs/source/user/index.rst

@@ -2,3 +2,17 @@
  Using Flake8
 ==============
 
+Flake8 can be used in many ways:
+
+- invoking it on the command-line
+
+- invoking it via Python
+
+- having it called by Git or Mercurial on or around committing
+
+This guide will cover all of these and the nuances for using Flake8.
+
+.. config files
+.. command-line tutorial
+.. VCS usage
+.. installing and using plugins