doc: Add external interface consistency guarantees

doc/README.md

@@ -56,6 +56,7 @@ The Bitcoin repo's [root README](/README.md) contains relevant information on th
 - [Translation Process](translation_process.md)
 - [Translation Strings Policy](translation_strings_policy.md)
 - [Travis CI](travis-ci.md)
+- [JSON-RPC Interface](JSON-RPC-interface.md)
 - [Unauthenticated REST Interface](REST-interface.md)
 - [Shared Libraries](shared-libraries.md)
 - [BIPS](bips.md)