Clarify MSRV strategy and versioning

[uklotzde]

• What is the reason behind choosing Rust 1.56 as the minimum supported version?
• Would it be possible to upgrade to a newer Rust version in a 1.x release?

[uklotzde]

error: package log v0.4.19 cannot be built because it requires rustc 1.60.0 or newer, while the currently active rustc version is 1.56.0

Let upgrade to Rust v1.60! -> #28

[uklotzde]

29 requires Rust 1.62