Mention in book/types#maps the storage issue with large maps

[anton-trunov]

We should also mention how to resolve it (using the blockchain as a map, essentially)

• Account state size's upper bound is around 65k cells, which, for instance, constraints a single contract map size to be about 20k - 30k entries
• using one large map does not allow to distribute workload hence making overall performance worse
• working with large maps makes 'compute fee' less deterministic