Add a limitation note in sgit

Namely that if the sed command has spaces it will be a problem. Whether
this will be fixed is yet to be seen.

bin/sgit

@@ -13,6 +13,10 @@
 # how it is and I don't think it would do good to add any additional features
 # except perhaps allowing for one to specify path to sed.
 #
+# A limitation is that if the sed command has spaces it will be a problem.
+# Whether this will be fixed is not yet decided. I have some ideas of how it
+# might happen but it might not be worth it.
+#
 # - Cody Boone Ferguson (@xexyl)
 #