Create a CONTRIBUTE.md introduction

[decryphe]

It would be helpful for anyone who wants to contribute if there's a quick intro into what to consider when working on rasn.

Especially a little checklist that mentions what should be done to run manual checks, e.g. cargo test, cargo verify, cargo clippy --all-targets.

[decryphe]

Here's the further learnings from my second contribution:

• cargo test --all-targets --workspace
• Using cross to run tests for other systems, e.g. 32-bit systems - cross test --target i686-unknown-linux-gnu --workspace

[Nicceboy]

There is new justfile that could help a bit: https://github.com/librasn/rasn/blob/main/justfile

Requires just.

To run all tests, run just all

To get available options

just --list
Available recipes:
    all       # Run the complete verification suite (formatting, linting, building, testing, documentation)
    build     # Build the entire workspace with proper target configuration, mode is determined by RELEASE_BUILD
    check     # Run all code quality checks (both formatting and linting)
    default   # Default entry point: Run all verification steps
    doc       # Build documentation and run documentation tests for each crate
    doc-build # Build documentation for the entire workspace
    doc-test  # Run documentation tests for each crate in the workspace (requires jq)
    fmt       # Verify code formatting compliance with default rustfmt standards
    lint      # Run Clippy for static code analysis with strict warning enforcement
    release   # Prepare a release build with complete documentation
    test      # Execute the test suite across the entire workspace (excludes documentation tests)
    toolchain # Set up or update the development environment with required Rust toolchain components