Add repo structure explanation to README.md

[Reinier van der Leer]

From feedback on the updated contribution guide:

<@kcze>

2. Is there anywhere a mention what's where in the repo (i.e. autogpt, forge, frontend, rnd dir), if not that would make it more clear where to contribute what.

6. "forge – the AutoGPT Forge SDK, located in forge/" .., autogpt is built using it. (without this it isn't obvious that some contributions should go there even if you want to change autogpt)

(message)

The README should be clear about what is where in the repo, and how the different subprojects relate to each other.