Minor fix in README.md page

Summary: It's not obvious that there are two different versions of the documentation.

Reviewed By: iseeyuan

Differential Revision: D56018543

fbshipit-source-id: 09e5facf3c2f2faaf216ebc76cd5c21697dbcb37

Author: mergennachin

README.md

@@ -18,7 +18,7 @@ Key value propositions of ExecuTorch are:
   capabilities such as CPUs, NPUs, and DSPs.
 
 For a comprehensive technical overview of ExecuTorch and step-by-step tutorials,
-please visit our [documentation website](https://pytorch.org/executorch).
+please visit our documentation website [for the latest release](https://pytorch.org/executorch/stable/index.html) (or the [main branch](https://pytorch.org/executorch/main/index.html)).
 
 ## Important: This is a preview release