Add common commands section in help text

Judging from user feedback coming out of Debian
distros, it appears that after installing `rustup`
from the package manager it is not immediately
clear which command installs the toolchain.
This patch adds a small text so that after
installation of `rustup`, which normally does not
invoke the toolchain installation for the user,
the user can still receive guidance on how to set
up the local toolchain immediately.

Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>

Author: dingxiangfei2009

src/cli/help.rs

@@ -11,7 +11,18 @@ pub(crate) fn rustup_help() -> String {
   for common platforms.
 
   If you are new to Rust consider running `rustup doc --book` to
-  learn Rust."
+  learn Rust.
+
+{HEADER}Common commands:{HEADER:#}
+
+  Update Rust toolchains and rustup
+
+    {LITERAL}$ rustup update{LITERAL:#}
+    
+  Install the current stable release of Rust for your host platform
+
+    {LITERAL}$ rustup toolchain install stable{LITERAL:#}
+  "
     )
 }