docs: Update cdk-mintd README to include prerequisites and system-wide install

[gillohner]

Description

Added prerequisites Rust and build dependencies pkg-config, protobuf-compiler to the cdk-mintd Readme

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Suggested CHANGELOG Updates

Minor Docs improvements

CHANGED

ADDED

REMOVED

FIXED

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Checklist

• I followed the code style guidelines
• I ran just final-check before committing

[thesimplekid]

I think we already have this in https://github.com/cashubtc/cdk/blob/main/DEVELOPMENT.md and it makes more sense there? But I'm sure we can still improve that document if you have recommendations.

[gillohner]

Whatever you think makes sense.

I was setting up a test-mint on a new Ubuntu-VM by looking at this document and though it might be useful to just add what I was missing to the Docs.

The Prerequisites may not be required here but I believe it might make sense to extend the build-section in the README with the install command and a check if it is working as cargo-mintd is implied as being installed system-wide in the section below it.