www.omnisharp.net contains old/invalid installation instructions for emacs

[razzmatazz]

The page at http://www.omnisharp.net/ contains old instructions on how to install the package on emacs.

• it refers to old server version, requires cloning, getting curl, etc. we don't need all that anymore (thankfully)
• doesn't have a note to M-x omnisharp-install-server
• doesn't have a note that there is spacemacs layer available

[willl]

When you're ready for it, send a PR to https://github.com/OmniSharp/OmniSharp.github.io.

[razzmatazz]

@willl -- uploaded, see https://github.com/OmniSharp/OmniSharp.github.io/pull/74