Document async vs. sync usage

[abrookins]

Redis OM Python supports both async and sync usage. Async support is provided in the aredis_om module, which is otherwise identical to the synchronous code found in redis_om. Document this.

[simonprickett]

Hold off on this until we move to the latest redis-py which includes full async support and makes aioredis deprecated.

[XChikuX]

There is no Redis Cluster support in the asynchronous module. Need to keep an eye on when that support gets added in the redis-py repo.