Add simple documentation of constants (#34)

* Add simple documentation of constants

Not sure if it's necessary to document every constant in README, so I simply mention the existence of `constants` and link it to the actual module.

* Document constants

* Fix spacing in document

README.md

@@ -95,6 +95,23 @@ A [bn.js](https://github.com/indutny/bn.js) object. Use `new BN(number)` to crea
 
 ---
 
+### constants
+A collection of useful [constants](src/constants.js).
+
+#### constants.ZERO_ADDRESS
+The initial value of a type `address` variable, i.e., `address(0)` in Solidity.
+
+#### constants.MAX_UINT256
+The maximum unsigned integer `2^256 - 1` represented in `BN`.
+
+#### constants.MAX_INT256
+The maximum signed integer `2^255 - 1` represented in `BN`.
+
+#### constants.MIN_INT256
+The minimum signed integer `-2^255` represented in `BN`.
+
+---
+
 ### ether
 Converts a value in Ether to wei.