Auto merge of #2103 - takkanm:add_install_help, r=alexcrichton

There was no "install" to help message.

src/bin/cargo.rs

@@ -51,6 +51,7 @@ Some common cargo commands are:
     bench       Run the benchmarks
     update      Update dependencies listed in Cargo.lock
     search      Search registry for crates
+    install     Install a Rust binary
 
 See 'cargo help <command>' for more information on a specific command.
 ";