Reword and clarify example.zsh custom file

custom/example.zsh

-# Add yourself some shortcuts to projects you often work on
-# Example:
+# You can put files here to add functionality separated per file, which
+# will be ignored by git.
+# Files on the custom/ directory will be automatically loaded by the init
+# script, in alphabetical order.
+
+# For example: add yourself some shortcuts to projects you often work on.
+#
+# brainstormr=~/Projects/development/planetargon/brainstormr
+# cd $brainstormr
 #
-# brainstormr=/Users/robbyrussell/Projects/development/planetargon/brainstormr
-#
\ No newline at end of file