Add llvm with wasm32-unknown-unknown as requirement to rust basic…

… bitcoin (#1017)

* Update README.md

* * [x]

rust/basic_bitcoin/README.md

@@ -26,6 +26,8 @@ For a deeper understanding of the ICP < > BTC integration, see the [Bitcoin inte
 * [x] Install the [IC
   SDK](https://internetcomputer.org/docs/current/developer-docs/setup/install/index.mdx).
   For local testing, `dfx >= 0.22.0-beta.0` is required.
+* [x] On macOS, `llvm` with the `wasm32-unknown-unknown` target (which is not included in the XCode installation by default) is required.
+To install, run `brew install llvm`.
 
 ## Step 1: Building and deploying sample code