Add README.md for the mastersrv, briefly explaining the setup

src/mastersrv/README.md

@@ -0,0 +1,55 @@
+mastersrv
+=========
+
+The mastersrv maintains a list of all registered DDNet (and Teeworlds) servers.
+Game servers try to register themselves with the mastersrv via an HTTP POST
+request on /ddnet/15/register. The mastersrv delays the registration until the
+the game server can prove it actually listens on the provided port
+(`need_challenge`). In order to do that, the mastersrv sends a UDP message
+containing an opaque token to the game server. The game server will include
+this token in its next register and the register process will succeed
+(`success`).
+
+To build the mastersrv, have a recent version of
+[Rust](https://www.rust-lang.org/) installed and run `cargo build --release` in
+the `src/mastersrv` folder. The resulting binary should appear in
+`src/mastersrv/target/release/mastersrv`.
+
+Use `--help` to see some of the options of the mastersrv.
+
+servers.json
+============
+
+The `servers.json` file produced by the mastersrv is not actually served
+automatically, it needs to be served by another HTTPS server, probably by the
+same as the reverse proxy handling HTTPS for the mastersrv. It is suggested to
+serve the `servers.json` file at the path `/ddnet/15/servers.json`.
+
+Logs
+====
+
+To enable logs, set the environment variable `RUST_LOG=mastersrv,info`. Logs
+should appear already when just starting the mastersrv.
+
+Reverse proxy
+=============
+
+In order to run this mastersrv in production, you must put it behind a reverse
+proxy that handles HTTPS, such as nginx, Caddy, or Apache.
+
+This means the mastersrv cannot determine the origin IP address by looking at
+the IP address of the request, it will come from the reverse proxy, likely on
+localhost. The mastersrv expects that the reverse proxy adds a header where it
+puts the original requester IP address, e.g. `Connecting-IP`. Add
+`--connecting-ip-header Connecting-IP` to the command line options of the
+mastersrv.
+
+For nginx, add `proxy_set_header Connecting-IP $remote_addr;`
+
+For Caddy, add `header_up Connecting-IP {http.request.remote}`.
+
+For Apache, add `RequestHeader set Connecting-IP "%{REMOTE_ADDR}s"` to the
+config.
+
+If you're behind Cloudflare, instead use `--connecting-ip-header
+CF-Connecting-IP` without any changes.