docs: filesystems: convert automount-support.txt to ReST

- Add a SPDX header;
- Add a document title;
- Adjust section titles;
- Mark literal blocks as such;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/ba7e2f2bf9aa2c7096772f5e7e8e609cb5fce07c.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index d993b33..373d695 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -25,6 +25,8 @@
    locking
    directory-locking
 
+   automount-support
+
    caching/index
 
    porting