Update README to explain preferred Arch build process

[mhegreberg]

I maintain the Arch AUR image for omnisharp-roslyn, and we dont use mono at all, we build directly with the dotnet CLI and MSbuild, by making a few tweaks to the build process.

I noticed that your readme still directs linux users to mono, so I thought I'd point them to our process that doesn't require mono.

[mhegreberg]

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