Mention minimum required dependent gem versions

[onyxraven]

When working with an existing project which has rubocop in its Gemfile, it needs to be a compatible version, or a user gets this error message:

Error running diagnostics: Incompatible RuboCop version. Ruby LSP requires >= 1.4.0

It looks like this comes from ruby-lsp but it doesn't mention it in readme/docs directly either.

[vinibispo]

@vinistock, Where would be a great place to put this info?

[vinistock]

I'd say we can put it in a new sub section under usage. Maybe call it implicit dependencies.

We only have one (RuboCop), but there's no existing place to put it, so I think it warrants a new section.

[andyw8]

Let's also add a comment in the code where we specify the minimum version, as a reminder to update the docs if we ever bump the minimum version..