Ready: Add section on Data Type contexts

- [x] Explanation
- [x] Ruby
- [x] Python
- [x] Erlang
- [x] Special section on the Java client
How about rather than "often essential to Riak" using "required by Riak". It is required, whether it's technically essential for a given step is an implementation detail. I only suggest it to get rid of the word "often", since we should be more precise if by dropping words like "sometimes", "usually", "generally", etc, where possible.