[FEATURE REQUEST]: Mention delta in readme

**Is your feature request related to a problem? Please describe.**
I saw references to the delta API in PRs and wanted to use `DeltaTable`, but could not resolve, since it exists in `Microsoft.Spark.Extensions.Delta` rather than the main package.

I then went to the readme and searched for `delta`, but couldn't find anything.
The nuget package link also redirects to the main package.
The landing page at https://dotnet.microsoft.com/apps/data/spark also doesn't mention it.

**Describe the solution you'd like**
Would it make sense to mention delta and / or the other packages in the readme?
I think this could improve discoverability.