docs: filesystems: convert fuse-io.txt to ReST

- Add a SPDX header;
- Add a document title;
- Add it to filesystems/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/88ec8025c1c5fc3ac5b65f1151c41ebcc696dc0e.1588021877.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/fuse-io.txt → Documentation/filesystems/fuse-io.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+Fuse I/O Modes
+==============
+
 Fuse supports the following I/O modes:
 
 - direct-io

Documentation/filesystems/index.rst

@@ -83,6 +83,7 @@ Documentation for filesystem implementations.
    hfsplus
    hpfs
    fuse
+   fuse-io
    inotify
    isofs
    nilfs2