Add a proper usage string

src/main.rs

@@ -12,6 +12,35 @@ use std::time::Instant;
 use borsh::BorshDeserialize;
 use borsh::BorshSerialize;
 
+const USAGE: &'static str = "Fastsync -- Transfer files over multiple TCP streams.
+
+Usage:
+  fastsync send <listen-addr> <in-files...>
+  fastsync recv <server-addr> <num-streams>
+
+Sender options:
+  <listen-addr>    Address (IP and port) for the sending side to bind to and
+                   listen for receivers. This should be the address of a
+                   Wireguard interface if you care about confidentiality.
+                   E.g. '100.71.154.83:7999'.
+
+  <in-files...>    Paths of files to send. Input file paths need to be relative.
+                   This is a safety measure to make it harder to accidentally
+                   overwrite files in /etc and the like on the receiving end.
+
+Receiver options:
+  <server-addr>    The address (IP and port) that the sender is listening on.
+                   E.g. '100.71.154.83:7999'.
+
+  <num-streams>    The number of TCP streams to open. For a value of 1, Fastsync
+                   behaves very similar to 'netcat'. With higher values,
+                   Fastsync leverages the fact that file chunks don't need to
+                   arrive in order to avoid the head-of-line blocking of a
+                   single connection. You should experiment to find the best
+                   value, going from 1 to 4 is usually helpful, going from 16
+                   to 32 is probably overkill.
+";
+
 const MAX_CHUNK_LEN: u64 = 4096 * 64;
 
 /// Metadata about all the files we want to transfer.
@@ -89,11 +118,7 @@ fn main() {
             let n_conn = &args[2];
             main_recv(addr, n_conn).expect("Failed to receive.");
         }
-        _ => {
-            eprintln!("Usage:");
-            eprintln!("  fastsync send <listen-addr> <in-files...>");
-            eprintln!("  fastsync recv <server-addr> <num-connections>");
-        }
+        _ => eprintln!("{}", USAGE),
     }
 }