Update readme to mention pin for 1.47

tcharding · tcharding · commit bef7992ce550 · 2023-03-19T12:19:40.000+11:00
We now require pinning for `serde` and `syn` if building with toolchain
1.47. Document this in the README.
diff --git a/README.md b/README.md
@@ -79,7 +79,7 @@ For more information please see `./CONTRIBUTING.md`.
 This library should always compile with any combination of features (minus
 `no-std`) on **Rust 1.41.1** or **Rust 1.47** with `no-std`.
 
-To build with the MSRV you will need to pin some dependencies:
+To build with the MSRV you will need to pin some dependencies (also for `no-std`):
 ```
 cargo update -p serde --precise 1.0.156
 cargo update -p syn --precise 1.0.107
