Clarify requirements

[robo9k]

Right now the README only mentions the need to install libmagic/file.

The documentation should specify version requirements and how to customize the cargo build to use a custom local installation of libmagic.

[robo9k]

#11 needs some documentation as well, which might just be copy pasting the pull request description into README.md → c9cf203

[robo9k]

v0.4.0 requires at least libmagic v5.39 which is documented in the crate README

There's no full description of how to use a custom local installation, but as can be seen from the GitHub workflows in this repo it boils down to running cargo

• with the PKG_CONFIG_PATH environment variable pointing to the directory of the custom libmagic.pc
• or with the VCPKG_ROOT env var pointing to the custom vcpkg root directory.