Add usage instructions to README (#69)

Author: JoshKarpel

README.md

@@ -8,5 +8,35 @@ A Befunge interpreter and text-UI IDE written in Rust.
 [`cargo`, the Rust package manager](https://doc.rust-lang.org/cargo/):
 
 ```bash
-$ cargo install fungoid
+cargo install fungoid
+```
+
+## Usage
+
+Fungoid provides a CLI command `fungoid`.
+Use the `--help` option to see what it can do:
+
+```console
+$ fungoid --help
+A Befunge interpreter and text-UI IDE written in Rust
+
+Usage: fungoid <COMMAND>
+
+Commands:
+  run       Run a program
+  ide       Start the TUI IDE
+  examples  Interact with the bundled example programs
+  help      Print this message or the help of the given subcommand(s)
+
+Options:
+  -h, --help     Print help
+  -V, --version  Print version
+```
+
+For example:
+
+```console
+$ fungoid examples run eratosthenes --profile
+Executed 4752 instructions in 213us 969ns (22,208,824 instructions/second)
+2357111317192329313741434753596167717379
 ```