Rework communication channels in the README.
Rework communication channels in the README.

* Easier to find links.
* Link to the new discord server.
* Link to MLIR discourse topic.

PiperOrigin-RevId: 301676408

iree-org/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,google/iree