docs: indicate incompatibility with experimental io-ts versions

Closes #446

Author: ericcrosson-bitgo

README.md

@@ -23,6 +23,11 @@ type-checked and runtime-validated HTTP calls in a standardized manner.
 [client]: packages/superagent-wrapper/README.md
 [server]: packages/express-wrapper/README.md
 
+## io-ts Compatibility
+
+**api-ts** is built and tested against the stable channel of io-ts (v2.1).
+**api-ts** is not currently compatible with the experimental channel of io-ts (v2.2).
+
 ## Developing
 
 ```sh