docs: add api documentation

README.md

@@ -34,6 +34,62 @@ try {
 }
 ```
 
+## API
+
+The API of this module is intended to be similar to the
+[Node.js `assert` module](https://nodejs.org/dist/latest/docs/api/assert.html).
+
+Each function will throw an instance of `HttpError` from
+[the `http-errors` module](https://www.npmjs.com/package/http-errors)
+when the assertion fails.
+
+### assert(value, [status], [message], [properties])
+
+Tests if `value` is truthy. If `value` is not truthy, an `HttpError`
+is thrown that is constructed with the given `status`, `message`,
+and `properties`.
+
+### assert.deepEqual(a, b, [status], [message], [properties])
+
+Tests for deep equality between `a` and `b`. Primitive values are
+compared with the Abstract Equality Comparison (`==`). If `a` and `b`
+are not equal, an `HttpError` is thrown that is constructed with the
+given `status`, `message`, and `properties`.
+
+### assert.equal(a, b, [status], [message], [properties])
+
+Tests shallow, coercive equality between `a` and `b` using the Abstract
+Equality Comparison (`==`). If `a` and `b` are not equal, an `HttpError`
+is thrown that is constructed with the given `status`, `message`,
+and `properties`.
+
+### assert.notDeepEqual(a, b, [status], [message], [properties])
+
+Tests for deep equality between `a` and `b`. Primitive values are
+compared with the Abstract Equality Comparison (`==`). If `a` and `b`
+are equal, an `HttpError` is thrown that is constructed with the given
+`status`, `message`, and `properties`.
+
+### assert.notEqual(a, b, [status], [message], [properties])
+
+Tests shallow, coercive equality between `a` and `b` using the Abstract
+Equality Comparison (`==`). If `a` and `b` are equal, an `HttpError` is
+thrown that is constructed with the given `status`, `message`, and
+`properties`.
+
+### assert.notStrictEqual(a, b, [status], [message], [properties])
+
+Tests strict equality between `a` and `b` as determined by the SameValue
+Comparison (`===`). If `a` and `b` are equal, an `HttpError` is thrown
+that is constructed with the given `status`, `message`, and `properties`.
+
+### assert.strictEqual(a, b, [status], [message], [properties])
+
+Tests strict equality between `a` and `b` as determined by the SameValue
+Comparison (`===`). If `a` and `b` are not equal, an `HttpError`
+is thrown that is constructed with the given `status`, `message`,
+and `properties`.
+
 ## Licence
 
 [MIT](LICENSE)