message: docs: update brief summary about rafs filesystem
  Let's give some hints about Rafs v6.
diff: @@ -26,7 +26,7 @@ Nydus takes in either [FUSE](https://www.kernel.org/doc/html/latest/filesystems/
  ![architecture](images/nydusd-arch.png)
  ## 2. Rafs
  -Rafs presents to users a userspace filesystem with separating filesystem's metadata with data. In a typical rafs filesystem, the metadata is stored in `bootstrap` while the data is stored in `blobfile`. Nydus splits container image into two parts, metadata and data, where metadata contains everything a container needs to start with, while data is stored in chunks with chunk size being 1MB.
  +Rafs is a filesystem image containing a separated metadata blob and several data-deduplicated content-addressable data blobs. In a typical rafs filesystem, the metadata is stored in `bootstrap` while the data is stored in `blobfile`. Nydus splits container image into two parts, metadata and data, where metadata contains everything a container needs to start with, while data is stored in chunks with chunk size being 1MB. Currently, Rafs has two versions called `Rafs v5` (which is a FUSE-based filesystem) and `Rafs v6` (which is compatible with the in-kernel EROFS filesystem). Note that, the following details are all about `Rafs v5` and meterials about `Raft v6` are still working in progress.
  ![rafs](./images/rafs-format.png)
  