Add example of mutable reference to shared resource
using borrow_mut and deref_mut.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>

At the moment cortex-m crate hides const versions of some functions,
including Mutex::new(), behind the const-fn feature. So this feature
needs to be enabled to cortex-m in Cargo.toml for shared resource
examples.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>

Address review comments by @andre-richter: proof-reading

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>