Add note about MSRV compile failure.

[xStrom]

This PR adds the note about how to deal with MSRV compilation failures. I optimistically didn't add it in #33 because the Rust toolchain can give basically the same instructions:

error: package `peniko v0.1.0` cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.67.1
Either upgrade to rustc 1.70 or newer, or use
cargo update -p [email protected] --precise ver
where `ver` is the latest version of `peniko` supporting rustc 1.67.1

However this nice error message is only given if rust-version is correctly specified by the dependency. In other cases it will be some much more obscure error about the specific code that can't be handled.

Given that we hide these instructions behind <details/> it won't cause too much noise and can be helpful in those obscure error scenarios.

[xStrom]

Turns out that on Linux the MSRV is actually 1.74 due to clap_derive v4.5.4. I did my initial MSRV checks on Windows, where parley can be compiled with 1.70. Other than that this first actual CI run was green.

[DJMcNab]

We really shouldn't be depending on clap_derive, but I've started the thread for that in Zulip