Add warning to top of readme

Author: TwitchBronBron

README.md

@@ -11,6 +11,8 @@ A superset of Roku's BrightScript language. Compiles to standard BrightScript.
 
 ## Overview
 
+**WARNING: this is the v0.66.0 branch with experimental features**
+
 The BrighterScript language provides new features and syntax enhancements to Roku's BrightScript language. Because the language is a superset of BrightScript, the parser and associated tools (VSCode integration, cli, etc...) work with standard BrightScript (.brs) files. This means you will get benefits (as described in the following section) from using the BrighterScript compiler, whether your project contains BrighterScript (.bs) files or not. The BrighterScript language transpiles to standard BrightScript, so your code is fully compatible with all roku devices.
 
 ## Features