Add minimal info and documentation on ProtoStream

I had to refresh my memory on what ProtoStream does and how to use it. The only info I found is the readme file on GitHub that only contains the project name and nothing else

It would be useful to at least add the main gola and main usage in the readme file as a first start.

But ideally, this project should have a landing page on http://infinispan.org/protostream and some info on the Infinispan wiki. It's more like shadowy source than open source if you don't push it to the outside world.