docs: document HLS_COPY_MAX_BITRATE

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -69,6 +69,7 @@ They should be defined where the binary is located or above it in an `.env` file
   skip them. Delete the `*.unsupported` files to force retries (for example
   after upgrading ffmpeg or adding libheif)
 - `VIDEO_PATH` is a path where HLS playlists and video parts should be stored
+- `HLS_COPY_MAX_BITRATE` (optional) is the highest source bitrate, in bits/s, that is stream-copied into HLS as-is. Higher-bitrate sources, and any with a short side over 1080 px, are transcoded down to 1080p [default: `20000000`]
 - `GIFS_DIRECTORY` is a path where generated video GIF thumbnails should be stored
 - `BIND_URL` is the url and port to bind to (typically your own IP address)
 - `LOCAL_BIND_URL` (optional) is a second, loopback listener [default: `localhost:8088`]. Set it to an empty string to skip it, e.g. to run a second instance such as the `dev/e2e` harness alongside one that already owns 8088