Update Readme with MDP/POMDP and Discrete/Continuous information

I think that a table with a basic problem description would greatly improve the package.

Something like the following:

### MDPs
problem | space | action
----------|----------|----------
GridWorld 3x3 | Discrete | Discrete

### POMDPS
problem | space | action | observation
----------|----------|----------|----------
MiniHallway | Discrete | Discrete | Discrete

I Will create a draft sometime in the future, I guess.

PS: This could also be extended with Deterministic / Stochastic actions / observations and state / action / observation space size.