Documentation/gitnamespaces.txt: cater to older asciidoc

Older asciidoc (e.g. 8.2.5 on Centos 5.5) is unhappy if a manpage does not
have a SYNOPSIS section. Show a sample (and a possibly bogus) command line
of running two commands that pay attention to this environment variable
with a customized value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Jamey Sharp <jamey@minilop.net>

Documentation/gitnamespaces.txt

@@ -5,6 +5,13 @@ NAME
 ----
 gitnamespaces - Git namespaces
 
+SYNOPSIS
+--------
+[verse]
+GIT_NAMESPACE=<namespace> 'git upload-pack'
+GIT_NAMESPACE=<namespace> 'git receive-pack'
+
+
 DESCRIPTION
 -----------