ATSAMD RTC monotonic referral in documentation (#1014)

* rtic-monotonics: Adds a section to the module documentation about ATSAMD chip monotonics provided in the `atsamd-hal` crate

* rtic-monotonics: Adds a section to README listing the supported microcontroller platforms.

* rtic-sync: Fix clippy complaints

error: the following explicit lifetimes could be elided: 'a
   --> rtic-sync/src/signal.rs:113:6
    |
113 | impl<'a, T: Copy> SignalReader<'a, T> {
    |      ^^                        ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
    |
113 - impl<'a, T: Copy> SignalReader<'a, T> {
113 + impl<T: Copy> SignalReader<'_, T> {

* xtask: Clippy fix warning

warning: elided lifetime has a name
   --> xtask/src/cargo_command.rs:427:14
    |
420 |     fn build_args<'i, T: Iterator<Item = &'i str>>(
    |                   -- lifetime `'i` declared here
...
427 |     ) -> Vec<&str> {
    |              ^ this elided lifetime gets resolved as `'i`
    |
    = note: `#[warn(elided_named_lifetimes)]` on by default

---------

Co-authored-by: Dan Whitman <daniel.whitman.2@us.af.mil>
Co-authored-by: Henrik Tjäder <henrik@tjaders.com>

xtask/src/cargo_command.rs

@@ -424,7 +424,7 @@ impl<'a> CargoCommand<'a> {
         features: &'i Option<String>,
         mode: Option<&'i BuildMode>,
         extra: T,
-    ) -> Vec<&str> {
+    ) -> Vec<&'i str> {
         let mut args: Vec<&str> = Vec::new();
 
         if nightly {