Add link to async SDK in documentation

**Context**

The [official stance](https://github.com/meilisearch/meilisearch-python/issues/414) for users needing async support is to use [sanders41/meilisearch-python-sdk](https://github.com/sanders41/meilisearch-python-sdk).

**Objective**

Update the `README` to include a section dedicated to async usage to link to the SDK.