More command examples

I had trouble finding the `toolchain uninstall` command, because I didn't realize that each subcommand had their own help pages to check on. This should hopefully make it clearer for somebody in the same situation.

Closes rust-lang#1362

Author: KasMA1990

README.md

@@ -519,6 +519,8 @@ Command | Description
 `rustup override set nightly-2015-04-01` | For the current directory, use a nightly from a specific date
 `rustup toolchain link my-toolchain "C:\RustInstallation"` | Install a custom toolchain by symlinking an existing installation
 `rustup show` | Show which toolchain will be used in the current directory
+`rustup toolchain uninstall nightly` | Uninstall a given toolchain
+`rustup toolchain help` | Show the `help` page for a subcommand (like `toolchain`) to see what actions are available on it
 
 ## Environment variables