Docs: Add information about portable RIDs

[weshaggard]

With .NET Core 2.0 we are moving towards using what we call portable RID's which are essentially RID's that aren't distro or version specific but instead contain assets that can work on lots of platforms. I think we need to refresh this document https://docs.microsoft.com/en-us/dotnet/core/rid-catalog to add information on the portable RIDs (i.e. linux-x64, win-x86/x64, osx-x64, etc).

cc @ericstj

[mairaw]

Should we also change examples where we show specific RIDs to use the portable ones? E.g. https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-publish

[weshaggard]

I think it is worth adding additional samples with the portable RIDs and the portable ones should be the primary example but the other RIDs should still have samples.

[yaakov-h]

Any news on this? The tooling is out, but the docs haven't been updated yet, which is plain confusing.

[mairaw]

Apologies on that @yaakov-h. This is coming soon.