Need to document how to enable LightningDb and Durable DData

[Aaronontheweb]

https://getakka.net/articles/clustering/distributed-data.html#settings - currently doesn't explain how to enable LightningDb and DData, which can be done via:

akka.net/src/contrib/cluster/Akka.DistributedData.Tests.MultiNode/DurableDataSpec.cs

Lines 41 to 47 in 978797a

	akka.cluster.distributed-data.durable.keys = [""durable*""]
	akka.cluster.distributed-data.durable.lmdb {{
	dir = """"""{tempDir}""""""
	map-size = 10 MiB
	write-behind-interval = {(writeBehind ? "200ms" : "off")}
	}}
	akka.cluster.distributed-data.durable.store-actor-class = ""Akka.DistributedData.LightningDB.LmdbDurableStore, Akka.DistributedData.LightningDB""

Need to explain how LightningDb works, why you'd want to use it with DData, what scenarios it covers, and how to enable it.