Add documentation to make it clear that decimal support is still experimental

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I was surprised to find that decimal support was causing the benchmark queries to return incorrect results. I think we need to make it clear that we are still working on decimal support so that users are not surprised. I even wonder if we should have a config flag to enable decimal support.

Describe the solution you'd like
Add documentation to make it clear that decimal support is still experimental

Describe alternatives you've considered
None

Additional context
None