Document the fact that we require protoc to build the bindings

Author: poljar

bindings/matrix-sdk-ffi/README.md

@@ -2,6 +2,14 @@
 
 This uses [´uniffi´](https://mozilla.github.io/uniffi-rs/Overview.html) to build the matrix bindings for native support and wasm-bindgen for web-browser assembly support. Please refer to the specific section to figure out how to build and use the bindings for your platform.
 
+# OpenTelemetry support
+
+The bindings have support for OpenTelemetry, allowing for the upload of traces to
+any OpenTelemetry collector. This support is provided through the
+[`opentelemetry`](https://docs.rs/opentelemetry/latest/opentelemetry/)
+crate, which requires the [`protoc`] binary to be installed during the build
+process.
+
 ## Platforms
 
 ### Swift/iOS sync
@@ -10,4 +18,4 @@ This uses [´uniffi´](https://mozilla.github.io/uniffi-rs/Overview.html) to bui
 
 ### Swift/iOS async
 
-TBD
+TBD