Fixes rust-lld errors when building example

Building the previous main.rs failed with: ERROR(cortex-m-rt): The interrupt vectors are missing.
Including the microbit crate in the example fixes this.

src/getting-started/01.00.BUILD.md

@@ -237,6 +237,7 @@ and cargo will automatically add `--target thumbv6m-none-eabi`.
 #![no_main]
 
 extern crate panic_halt;
+extern crate microbit;
 
 use cortex_m_rt::entry;
 
@@ -272,6 +273,7 @@ An easy way to implement this is to use an infinite loop.
 #![no_main]
 
 extern crate panic_halt;
+extern crate microbit;
 
 use cortex_m_rt::entry;