Update top-level README.md to recommend using a release tag (#912)

Summary:
Pull Request resolved: #912

Without this, users might try using the `main` branch, which we can't guarantee quality for.

Reviewed By: mergennachin

Differential Revision: D50288620

fbshipit-source-id: a4414f95e14b14e5fb4a97c07d09981497e194f5

Author: dbort

README.md

@@ -30,6 +30,12 @@ and feedback about ExecuTorch using the tag **#executorch** and
 our [GitHub repository](https://github.com/pytorch/executorch/issues)
 for bug reporting.
 
+The ExecuTorch code and APIs are still changing quickly, and there are not yet
+any guarantees about forward/backward source compatibility. We recommend using
+the latest `v#.#.#` release tag from the
+[Releases](https://github.com/pytorch/executorch/releases) page when
+experimenting with this preview release.
+
 ## Directory Structure [WIP]
 
 ```