Add option to skip manifest guide in cargo-new

[pbzweihander]

Every Cargo.toml file generated with cargo new command includes the following line:

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

I think it would be great if I can choose to remove this line.

This PR adds an option not to include the guide.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

Please see the contribution instructions for more information.

[ehuss]

Thanks for the PR! However, I would prefer to not add individual options to control the generated project, but instead to pursue templates which would allow people to customize projects however they want. In the meantime, there are tools such as cargo-generate or kickstart which can create customized projects.