Explain the usage of SmallBigInt.js

[cyan-2048]

this file seems to be a monkeypatch for JSBI to use Native BigInt instead of pure javascript big ints, could there be a better explanation on the README?

if my assumption is correct, SmallBigInt could be very useful for certain use cases.

[Yaffle]

when the code is transformed using babel-plugin-transform-bigint it uses JSBI, I want to replace JSBI with a wrapper around native BigInts as it native bigint implementation is faster. Another think, is there is SmallBigInt class with same API as JSBI, but which stores small values as numbers, which is also faster in some cases