docs: filesystems: add overlayfs to index.rst

While the document is there, it is currently missing at the
index file.

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/3b8e7783b1fcc71e4f94af5ea8e5fa264392f8c4.1580193653.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <[email protected]>

Author: mchehab

Documentation/filesystems/index.rst

@@ -47,5 +47,6 @@ Documentation for filesystem implementations.
    :maxdepth: 2
 
    autofs
+   overlayfs
    virtiofs
    vfat