Add no-outside-dependency guideline

Signed-off-by: Steven Barth <steven@midlink.org>

README

@@ -17,6 +17,7 @@ Please make sure that all packages you commit or request to pull:
 	* Package a version which is still maintained by the upstream author.
         * Have yourself or another person listed in the (PKG_)MAINTAINER field.
 	* Will be updated regularly to maintained and supported versions. 
+	* Have no dependencies outside the openwrt core packages or this feed.
 
 Please sign-off your commits / pull requests to this repository. See:
 https://dev.openwrt.org/wiki/SubmittingPatches#a10.Signyourwork