Add a usage example to the README

Author: mkantor

README.md

@@ -1,3 +1,60 @@
 # parsing
 
 A [parser combinator](https://en.wikipedia.org/wiki/Parser_combinator) library.
+
+## Usage Example
+
+A simple calculator for addition and subtraction of natural numbers:
+```ts
+import either from '@matt.kantor/either'
+import {
+  lazy,
+  literal,
+  map,
+  oneOf,
+  oneOrMore,
+  sequence,
+  type Parser,
+} from '@matt.kantor/parsing'
+
+const operator = oneOf([literal('+'), literal('-')])
+
+const number = map(
+  oneOrMore(
+    oneOf([
+      literal('0'),
+      literal('1'),
+      literal('2'),
+      literal('3'),
+      literal('4'),
+      literal('5'),
+      literal('6'),
+      literal('7'),
+      literal('8'),
+      literal('9'),
+    ]),
+  ),
+  Number,
+)
+
+const compoundExpression = map(
+  sequence([number, operator, lazy(() => expression)]),
+  ([a, operator, b]) => {
+    switch (operator) {
+      case '+': return a + b
+      case '-': return a - b
+    }
+  },
+)
+
+const expression: Parser<number> = oneOf([compoundExpression, number])
+
+const evaluate = (input: string) =>
+  either.flatMap(expression(input), ({ remainingInput, output }) =>
+    remainingInput.length !== 0
+      ? either.makeLeft('excess content followed valid input')
+      : either.makeRight(output),
+  )
+
+console.log(evaluate('2+2-1').value) // logs "3"
+```