[Chronik] Add Rust install instructions to CONTRIBUTING.md

Summary: Instructions are for installing the latest stable Rust version and for installing the specific nightly used for Chronik formatting.

Test Plan:

1. Execute the instructions in a clean environment (e.g. ubuntu/debian docker)
2. Running rustc --version prints the latest stable Rust version
3. Running rustc +abc-nightly --version prints the installed nightly version + date (2022-06-28)

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11705