Remove "WIP" from top-level README file (#932)

Summary:
Pull Request resolved: #932

Let's remove it. Having WIP on the top README file just looks sloppy right away.

Reviewed By: dbort

Differential Revision: D50308053

fbshipit-source-id: cbb730f05a6f8f65cca039a7f75326314c0eda88

Author: mergennachin

README.md

@@ -36,7 +36,7 @@ the latest `v#.#.#` release tag from the
 [Releases](https://github.com/pytorch/executorch/releases) page when
 experimenting with this preview release.
 
-## Directory Structure [WIP]
+## Directory Structure
 
 ```
 executorch