proper documentation for nimble

[stoneface86]

I think that nimble should have a documentation page that isn't this github's README.md. Since it is shipped with nim, there should be a page for nimble at https://nim-lang.org/docs/tools.html.

Now that Nim's documentation generation supports markdown, perhaps the existing README.md could be used to generate the page.

I personally find reading the current documentation via github a pain.