[DOCS]: Update README to include new features (like /link_to) #184

Commands (and other features) like the one linking to the server site and the one that allows for LaTeX formatting have newly emerged, but the README has not been updated to include them. It would be helpful to do so, especially since slash commands are not included in >help either.

There is also the admin panel pending! If #180 is merged, it could be helpful to also include that in the README.