README: add example usage.

README.md

@@ -5,7 +5,38 @@ its own HDL depending on how much I love Scala. :)
 
 ## Example
 
-<https://github.com/kivikakk/sevsegsim>.
+<https://github.com/kivikakk/sevsegsim>
+
+```console
+$ sbt run
+sevsegsim 0.1.0 (Chryse 0.1.0-SNAPSHOT)
+  -h, --help      Show help message
+  -v, --version   Show version of this program
+
+Subcommand: build
+Build the design, and optionally program it.
+  -b, --board  <arg>         Board to build for.  Choices: icebreaker
+  -F, --full-stacktrace      Include full Chisel stacktraces
+      --no-full-stacktrace
+  -p, --program              Program the design onto the board after building
+      --no-program
+  -h, --help                 Show help message
+
+Subcommand: cxxsim
+Run the C++ simulator tests.
+  -c, --compile       Compile only; don't run
+      --no-compile
+  -d, --debug         Generate source-level debug information
+      --no-debug
+  -O, --optimize      Build with optimizations
+      --no-optimize
+  -v, --vcd  <arg>    Output a VCD file when running cxxsim (passes --vcd <file>
+                      to the executable)
+  -h, --help          Show help message
+
+ trailing arguments:
+  trailing (not required)   Other arguments for the cxxsim executable
+```
 
 ## Quick feature overview