monotonics add readme (#1012)

* rtic-monotonics: Add README

* rtic-monotonics: Add links to changelog

rtic-monotonics/README.md

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+[![crates.io](https://img.shields.io/crates/v/rtic-monotonics.svg)](https://crates.io/crates/rtic-monotonics)
+[![crates.io](https://img.shields.io/crates/d/rtic-monotonics.svg)](https://crates.io/crates/rtic-monotonics)
+
+# `rtic-monotonics`
+
+> Core abstractions of the Real-Time Interrupt-driven Concurrency (RTIC) Monotonics timers
+
+`rtic-monotonics` is for RTIC v2.
+
+For RTIC v1 see [`rtic-monotonic`](https://github.com/rtic-rs/rtic-monotonic)
+
+## [Documentation](https://docs.rs/rtic-monotonics)
+
+[RTIC book: chapter on monotonics](https://rtic.rs/2/book/en/by-example/delay.html)
+
+### [Changelog `rtic-monotonics`](https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics/CHANGELOG.md)
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+## Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.