Add value precision workaround to README.md

[hqm42]

I encountered rounding errors while summing up large numbers with rollup:

[1] pry(main)> MyModel.rollup('my rollup') { |r| r.sum(:my_int_column) }
[2] pry(main)> Rollup.series('my rollup').values.sum == MyModel.pluck(:my_int_column).sum
=> false