Update README.md

Author: gmac

subservice-languages/README.md

@@ -1,6 +1,6 @@
 # Subservice languages
 
-A stitched gateway is unconcerned with what programming language(s) subservices use as long as they can provide an annotated SDL and fulfill their schema contract. The examples below demonstrate setting up stitching SDLs using a variety of languages and tools.
+A stitched gateway that is [configured by SDL directives](https://www.graphql-tools.com/docs/stitch-directives-sdl) is unconcerned with what programming language(s) subservices use as long as they can provide an annotated SDL and fulfill that schema contract. The examples below demonstrate setting up stitching SDLs using a variety of languages and tools.
 
 When using [static JavaScript stitching config](https://www.graphql-tools.com/docs/stitch-type-merging), subservices don't even need to provide an annotated SDL, at which time no special subservice setup is necessary.