Add docs about sharding to the cli module

src/cli.rs

@@ -46,6 +46,11 @@
 //! $ cargo run -- start [TOKEN]
 //! ```
 //!
+//! To run multiple shards, pass the `--shard` or `-s` argument to the `start` command with the
+//! shard ID and total amount of shards as `id:total`. For example, for a single-shard bot, you'd
+//! set it to `-s 0:1`. ID is zero-indexed (0 means first shard), total is one-indexed (0 means no
+//! shards).
+//!
 //! # Running the Documentation Server
 //!
 //! Payphone has a built-in documentation server that serves the project's documentation and