Document less-than, equals, and compare

Author: jessealama

README.md

@@ -12,6 +12,8 @@ This library is a prototype for the [decimal proposal](https://github.com/tc39/p
 -   remainder (`remainder`)
 -   rounding (`round`)
 -   `toString` emitting both decimal and exponential syntax (default is decimal)
+-   less-than (`lessThan`) and equals (`equals`) to compare by mathematical value
+-   `compare` for comparing Decimals as digit strings (e.g. `1.2` < `1.20`)
 
 ## Comparisons