Update README for Code Lens features

[andyw8]

Covering the new functionality added in #67.

[vinistock]

These features are all listed as part of our documentation generated by RDoc. Should we link to there instead to avoid repeating ourselves?

[andyw8]

We could, but it's not the most appealing format, e.g. https://shopify.github.io/ruby-lsp-rails/RubyLsp/Rails/Hover.html

Maybe the README can act as a brief summary of the behaviour, and we can link to the docs for more detail? That's something I commonly see for other gem's READMEs.

[vinistock]

Let's at least limit the README to a few examples. Otherwise, we're documenting the same thing twice, which increases the likelihood we'll forget to update the documentation if anything changes.

[vinistock]

Let's at least limit the README to a few examples. Otherwise, we're documenting the same thing twice, which increases the likelihood we'll forget to update the documentation if anything changes.