docs: explain axios dependency in README

There's probably a better way of handling this in rollup.

README.md

@@ -10,6 +10,12 @@ Implements:
 - validation of messages against a specified OpenAPI schema 
 - decode/encode stubs
 
+## Dependencies 
+
+An axios library. Axios is included as a dev dependency to run the tests. 
+I've not installed it as a runtime dependency because in nuxt you need to 
+use @nuxtjs/axios instead.
+
 ## Install
 
 ### Yarn