doc: note that our API has diverged from upstream

Author: fanquake

README.md

@@ -15,7 +15,7 @@ This class is aligned with the JSON standard, [RFC
 ## Library usage
 
 This is a fork of univalue used by Bitcoin Core. It is not maintained for usage
-by other projects. Notably, the API may break in non-backward-compatible ways.
+by other projects. Notably, the API is broken in non-backward-compatible ways.
 
 Other projects looking for a maintained library should use the upstream
 univalue at https://github.com/jgarzik/univalue.