Stop recommending adding `register` to config.nu

I didn't realize that plugin registration is persisted between runs of
nushell in the `plugin.nu` config file (`$nu.plugin-path`)

Because of this, `register` really only needs to be run when you install
or update the plugin.

Adding plugin registrations to `config.nu` leads to longer startup times
and is probably best avoided

README.md

···
 
 ## Install with Cargo
 
-Run: `cargo install --locked nu_plugin_dbus`
+From within nushell:
 
-Then add `register ~/.cargo/bin/nu_plugin_dbus` to your `~/.config/nushell/config.nu`
+    cargo install --locked nu_plugin_dbus
+    register ~/.cargo/bin/nu_plugin_dbus
 
 ## Usage