Document full list of added and NOT added features

[nsaulnier-ti]

Important to list features that do exist, and things that do not currently exist in the simulator.

Initial need is a list of what currently exists and doesn't exist. Before going public, we also should clarify what features are planned for the future, and which features will not get added in the future. The future-facing decisions need some more verbal discussions.

e.g., INTC block, IEP timer, etc.

[nsaulnier-ti]

also list missing features like TX_PRU, being able to simulate more than 2 cores at once, etc.