Add libwebrtc/README.md explaining how to build and use it (#499)

webrtc-sys/libwebrtc/README.md

@@ -0,0 +1,28 @@
+This directory can contain a checkout of WebRTC. The build scripts
+here will install dependencies, checkout the version that LiveKit
+currently uses, apply some patches to it, and build it. For example to
+do a Linux debug build on x64:
+
+```sh
+$ ./build-linux.sh --arch x64 --profile release
+```
+
+After running this, `linux-x64-debug/lib/libwebrtc.a` should
+exist. This can be rerun to rebuild it, but will complain about
+patches not applying as they have already been applied.
+
+If something goes wrong it may be helpful to consult the [WebRTC native
+development documentation](https://webrtc.googlesource.com/src/+/main/docs/native-code/development/).
+
+# Building LiveKit Rust SDK with custom WebRTC checkout
+
+Add the following environment variable to `/.config/config.toml`, to
+specify use of a custom WebRTC build:
+
+```toml
+[env]
+LK_CUSTOM_WEBRTC = { value = "webrtc-sys/libwebrtc/linux-x64-release", relative = true }
+```
+
+Note that `linux-x64-debug` should be replaced with the artifact
+directory appropriate for your configuration.