docs: add --help message to README.md

README.md

@@ -1,12 +1,57 @@
 DayZ Linux workshop setup
 ====
 
-This is an expermental and still work-in-progress setup+launcher script for workshop mods of DayZ standalone when running the game via Proton on Linux.
+This is an experimental and still work-in-progress setup+launcher script for workshop mods of DayZ standalone when running the game via Proton on Linux.
 
 Proton is currently unable to start the game's own regular launcher application which sets up mods and launch parameters for the game client. The game however does work fine when launching the client directly, so mods can be set up and configured manually, which is what this script tries to do, similar to what the launcher would do.
 
 ## Usage
 
+```
+Usage: dayz-mods.sh [OPTION]... [MODID]...
+
+Automatically set up mods for the DayZ client
+and print the game's -mod command line argument.
+
+Command line options:
+
+  -h
+  --help
+    Print this help text.
+
+  -d
+  --debug
+    Print debug messages to output.
+
+  -l
+  --launch
+    Launch DayZ after resolving and setting up mods
+    instead of printing the game's -mod command line argument.
+
+  -s <address[:port]>
+  --server <address[:port]>
+    Retrieve a server's mod list and add it to the remaining input.
+    Uses the daemonforge.dev DayZ server JSON API.
+    If --launch is set, it will automatically connect to the server.
+
+  -p <port>
+  --port <port>
+    The server's query port (not to be confused with the server's game port).
+    Default is: 27016
+
+Environment variables:
+
+  STEAM_ROOT
+    Set a custom path to Steam's root directory. Default is:
+    ${XDG_DATA_HOME:-${HOME}/.local/share}/Steam
+    which defaults to ~/.local/share/Steam
+
+    If the game is stored in a different Steam library directory, then this
+    environment variable needs to be set/changed.
+```
+
+## Examples
+
 ```sh
 ./dayz-mods.sh MODID1 MODID2 MODID3...
 ./dayz-mods.sh --server ADDRESS