docs: filesystems: convert hfsplus.txt to ReST

Just trivial changes:

- Add a SPDX header;
- Add it to filesystems/index.rst.

While here, adjust document title, just to make it use the same
style of the other docs.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4298409da951fbee000201a6c8d9c85e961b2b79.1581955849.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/hfsplus.txt → Documentation/filesystems/hfsplus.rst

+.. SPDX-License-Identifier: GPL-2.0
 
+======================================
 Macintosh HFSPlus Filesystem for Linux
 ======================================
 

Documentation/filesystems/index.rst

@@ -67,6 +67,7 @@ Documentation for filesystem implementations.
    f2fs
    gfs2
    gfs2-uevents
+   hfsplus
    fuse
    overlayfs
    virtiofs