Document why to use Marksman instead of VSCode's built-in markdown LSP

[waldyrious]

I just came across Marksman and searched the issues and documentation both here and in the main repository, but couldn't find an overview of how Marksman differs from VSCode's default LSP, and therefore why one might want to use Marksman when VSCode already comes with markdown support out of the box.

A list of pros and cons (if any 🙂) in this repo's README would be great! Alternatively, a section in the Marksman repo's Features doc could work too, or even just a succinct entry in the FAQ.

[artempyanykh]

I haven't used VSCode's markdown support in a while, so can't comment extensively. If you've used both and have thoughts, feel free to put them into a PR 🙂