Add config example with respect to branch

Update config.txt with example with respect to branch
config variable. This give a better idea regarding
how branch names are expected.

Signed-off-by: Aneesh Kumar K.V <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: kvaneesh

Documentation/config.txt

@@ -31,6 +31,11 @@ Example
 		external = "/usr/local/bin/gnu-diff -u"
 		renames = true
 
+	[branch "devel"]
+		remote = origin
+		merge = refs/heads/devel
+
+
 Variables
 ~~~~~~~~~