Description of changes:

Simplifies/standardizes the README.md file. The rationale:

• In description: Replaces the old description to make it more concrete and clear. The new description is inspired by CBMC's description and adds "- even in unsafe Rust code". Rewording are welcome (I even considered converting into a bulleted list, let me know if you like this idea).
• Simplifies the text for installation.
• Removes "What does Kani do?" because that should be clear in the description. We can add a "Documentation" section later if you like but most people will have arrived to it already by then.
• Comments on the code snippet may help readers to understand the example. A couple of rewording being made here too.
• Replaces "Developer guide" with "Contributing", which is the standard.
• In license: Replaced "the Rust compiler" with "Rust". This is important: We use code from the Rust project in general (e.g., rustdoc), not just the compiler.

Resolved issues:

Part of #700

Testing:

• How is this change tested? N/A

• Is this a refactor change? No.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.