Add a note about env vars to the super prompt.

bash/super_prompt.sh

@@ -1,6 +1,9 @@
 # Produces a nice, colorful 2-line prompt of the form:
 # [username@hostname:current_directory] (git_repo|branch)
 # $
+#
+# You can add the output of arbitrary env vars with something like this:
+# ${C_RED}\$(echo -n \$ENV_VAR_NAME)${C_NO}
 
 src_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source $src_dir/colorize.sh