[BigIntModule] Update README.

Author: xwu

Sources/BigIntModule/README.md

@@ -1,20 +1,33 @@
 # Arbitrarily Large Signed Integers (BigInt)
 
-This module provides a `BigInt` number type:
+This module provides a `BigInt` number type to represent arbitrarily large signed integers:
 
 ```swift
 1> import BigIntModule
 2> let a = 42 as BigInt
 ```
 
-The usual arithmetic operators are provided, as well as conversion to and from a collection of words (of type `UInt`), plus conformances to the obvious usual protocols: `Comparable` (hence also `Equatable`), `Hashable`, `LosslessStringConvertible`, `Codable`, `ExpressibleByIntegerLiteral`, and `SignedInteger` (hence also `Strideable`, `AdditiveArithmetic`, `Numeric`, `SignedNumeric`, and `BinaryInteger`).
+The usual arithmetic operators are provided, as well as conversion to and from a collection of words (of type `UInt`) or a string (in any base between 2 and 36), plus conformances to the obvious usual protocols:
+
+* `Comparable` (hence also `Equatable`)
+* `Hashable`
+* `ExpressibleByIntegerLiteral`
+* `SignedInteger` (hence also `BinaryInteger`, `Strideable`, `SignedNumeric`, `Numeric`, and `AdditiveArithmetic`)
+* `LosslessStringConvertible`
+* `Codable`
+
+## Implementation notes
+
+Internally, `BigInt` represents values by their signum function, significand, and exponent using the following notional formula:
+
+```swift
+let value = signum * significand << (UInt.bitWidth * exponent)
+```
+
+Bitwise and bit shift operators use the two's complement representation of negative values despite the way in which those values are stored internally.
 
 ### Dependencies:
 - None.
 
 ### Test dependencies:
 - `attaswift/BigInt` (reference implementation).
-
-## Design notes
-
-_To be completed_