git-prompt.sh: document GIT_PS1_STATESEPARATOR

The environment variable GIT_PS1_STATESEPARATOR can be used to set the
separator between the branch name and the state symbols in the prompt.

At present the variable is not mentioned in the inline documentation which
makes it difficult for the casual user to identify.

Signed-off-by: Joe Cridge <joe.cridge@me.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -61,6 +61,10 @@
 #     git           always compare HEAD to @{upstream}
 #     svn           always compare HEAD to your SVN upstream
 #
+# You can change the separator between the branch name and the above
+# state symbols by setting GIT_PS1_STATESEPARATOR. The default separator
+# is SP.
+#
 # By default, __git_ps1 will compare HEAD to your SVN upstream if it can
 # find one, or @{upstream} otherwise.  Once you have set
 # GIT_PS1_SHOWUPSTREAM, you can override it on a per-repository basis by