docs: specify the source of truth for the spec (asyncapi#781)

Author: char0n

README.md

@@ -39,6 +39,7 @@
 ## Read the specification
 
 The latest draft specification can be found at [spec/asyncapi.md](./spec/asyncapi.md) which tracks the latest commit to the master branch in this repository.
+The human-readable markdown file is the source of truth for the specification.
 
 - [Version 2.4.0](https://github.com/asyncapi/spec/blob/v2.4.0/spec/asyncapi.md) (latest)
 - [Version 2.3.0](https://github.com/asyncapi/spec/blob/v2.3.0/spec/asyncapi.md)