update docs (#360)

README.md

@@ -143,7 +143,7 @@ await fastify.register(import('@fastify/rate-limit'), {
 - `global` : indicates if the plugin should apply rate limiting to all routes within the encapsulation scope.
 - `max`: maximum number of requests a single client can perform inside a timeWindow. It can be an async function with the signature `async (request, key) => {}` where `request` is the Fastify request object and `key` is the value generated by the `keyGenerator`. The function **must** return a number.
 - `ban`: maximum number of 429 responses to return to a client before returning 403 responses. When the ban limit is exceeded, the context argument that is passed to `errorResponseBuilder` will have its `ban` property set to `true`. **Note:** `0` can also be passed to directly return 403 responses when a client exceeds the `max` limit.
-- `timeWindow:` the duration of the time window. It can be expressed in milliseconds or as a string (in the [`ms`](https://github.com/zeit/ms) format)
+- `timeWindow:` the duration of the time window. It can be expressed in milliseconds, as a string (in the [`ms`](https://github.com/zeit/ms) format), or as an async function with the signature `async (request, key) => {}` where `request` is the Fastify request object and `key` is the value generated by the `keyGenerator`. The function **must** return a number.
 - `cache`: this plugin internally uses a lru cache to handle the clients, you can change the size of the cache with this option
 - `allowList`: array of string of ips to exclude from rate limiting. It can be a sync or async function with the signature `(request, key) => {}` where `request` is the Fastify request object and `key` is the value generated by the `keyGenerator`. If the function return a truthy value, the request will be excluded from the rate limit.
 - `redis`: by default, this plugin uses an in-memory store, but if an application runs on multiple servers, an external store will be needed. This plugin requires the use of [`ioredis`](https://github.com/redis/ioredis).<br> **Note:** the [default settings](https://github.com/redis/ioredis/blob/v4.16.0/API.md#new_Redis_new) of an ioredis instance are not optimal for rate limiting. We recommend customizing the `connectTimeout` and `maxRetriesPerRequest` parameters as shown in the [`example`](https://github.com/fastify/fastify-rate-limit/tree/master/example/example.js).