Update main repository README after integrating 3i into the main codebase

[Yaxuan-w]

After integrating 3i into the main codebase, we need to update the main repository’s README to reflect this change. I’m planning to add threei to the “In-house Projects” section, but it’s still unclear whether we should also include other core components such as cage, typemap, and rawposix for consistency. @rennergade

[rennergade]

Lets definitely briefly describe them elsewhere in the docus. For the README we should mention them but not go into detail, just point to where that is.