Add basic info about .NET Standard in docs

[jeroenheijmans]

I was looking for information around Mono's support for .NET Standard, but couldn't find any. I would've expected it to be on the docs page under "About Mono" (or possibly "FAQ" or "Advanced Topics") but found nothing there.

I've also checked the Versioning docs and a few other specific top level pages but found little to no info. My Google-fu only leads to a few specific version's release notes mentioning it off-hand.

It would be nice if there is a page e.g. under "About" explaining Mono's position within the new .NET Core / .NET Standard ecosystem.

[akoeplinger]

The most up to date info is http://www.mono-project.com/docs/about-mono/releases/5.2.0/#net-standard-20-groundwork.

I'll see about adding it to the About Mono page, thanks for the suggestion!