Add usage to README

Author: qzed

README.md

@@ -3,6 +3,28 @@
 Control various aspects of Microsoft Surface devices on Linux from the Command-Line.
 Aims to provide a unified front-end to the various sysfs-attributes and special devices.
 
+## Usage
+
+```
+USAGE:
+    surface <SUBCOMMAND>
+
+FLAGS:
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+
+SUBCOMMANDS:
+    dgpu           Control or query the dGPU power state
+    help           Prints this message or the help of the given subcommand(s)
+    latch          Control the latch/dtx-system on the Surface Book 2
+    performance    Control or query the current performance-mode
+    status         Query the current system status
+```
+
+See `surface <subcommand> help` for more details.
+
+_Hint:_ You can specify the subcommand by any unabiguous prefix of it, i.e. `surface perf` and `surface p` will both evaluate to `surface performance`.
+
 ## Building from Source
 
 Building this application from source follows the standard rust procedure, i.e. simply call `cargo build --release --locked` for a release-ready executable.