Add top-level README.md

Signed-off-by: Bo Chen <bchen@crusoe.ai>

Author: likebreath

README.md

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+# iommufd
+
+The `iommufd` workspace hosts libraries related to Rust bindings and
+wrappers to the
+[IOMMUFD](https://docs.kernel.org/userspace-api/iommufd.html) subsystem
+from the Linux kernel. It currently consists of the following crates:
+
+- `iommufd-bindings` -> Rust FFI bindings to IOMMUFD generated using bindgen
+- `iommufd-ioctls` -> Safe wrappers over IOMMUFD uAPIs