all: discourage use of DecimalField until Spanner has NUMERIC

[odeke-em]

Coming here from the design doc, with support and advisory from @timgraham and @bvandiver, let's document in the README and also perhaps fail in code if users try to use DecimalField, up until Cloud Spanner announces support for NUMERIC.

[timgraham]

We can use the system check framework to raise an error if any models use DecimalField.

[odeke-em]

Sounds like a plan to me, thank you!