Added a "recommended reading" section

Hello,

That's a first (and shy) PR.
Web resources are totally great, and I think most users will prefer to use them. However there is some literature we can't totally skip (not quote at least).

I propose to add a "Recommended Reading" section, with 3 firsts proposals:
 - DDD by Eric Evans, which was the "founding" book
 - IDDD By Vaughn Vernon, which has the enormous advantage of being practical (discussing about implementation) and recent, so it's really a good reading
 - P of EAA by Martin Fowler describes many patterns/structures which are pretty common in DDD (not to say core), such as the "Domain Model", "Repository", "Value object", some useful such as "Layer Supertype"