Platform and compatabillity

[rogeralsing]

We should add some doc page that describes where and under what conditions we can run Akka.NET.
Listing each module and where it works.

Sort of:

Module	.NET	Mono *nix	Mono RPI
Akka Core	✅	✅	✅
Akka.Remote	✅	✅	✅
Akka.Cluster	✅	✅	???
Akka.Persistence.SQL	✅	No	No

[Danthar]

I think its also important to note which .NET versions are supported. For example due to some recent async related fixes I believe we are firmly linked to the 4.5 runtime. Which could be an important detail for some folks.