Update README.md

Author: paulyoung

README.md

@@ -1,6 +1,8 @@
 # cdk-swift
 
-Swift Canister Development Kit for the Internet Computer.
+<picture><img src="https://img.shields.io/badge/status%EF%B8%8F-proof%20of%20concept-blueviolet"></picture>
+
+(Proof of concept) Swift Canister Development Kit for the Internet Computer.
 
 ## Usage
 
@@ -20,3 +22,6 @@ To debug Wasm in the browser:
 ```
 python3 -m http.server 1234
 ```
+
+## Known Issues
+* Importing `Foundation` produces a Wasm binary that exceeds the file size limit for deployment.