docs: filesystems: convert btrfs.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>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/1ef76da4ac24a9a6f6187723554733c702ea19ae.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/btrfs.rst

@@ -0,0 +1,34 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====
+BTRFS
+=====
+
+Btrfs is a copy on write filesystem for Linux aimed at implementing advanced
+features while focusing on fault tolerance, repair and easy administration.
+Jointly developed by several companies, licensed under the GPL and open for
+contribution from anyone.
+
+The main Btrfs features include:
+
+    * Extent based file storage (2^64 max file size)
+    * Space efficient packing of small files
+    * Space efficient indexed directories
+    * Dynamic inode allocation
+    * Writable snapshots
+    * Subvolumes (separate internal filesystem roots)
+    * Object level mirroring and striping
+    * Checksums on data and metadata (multiple algorithms available)
+    * Compression
+    * Integrated multiple device support, with several raid algorithms
+    * Offline filesystem check
+    * Efficient incremental backup and FS mirroring
+    * Online filesystem defragmentation
+
+For more information please refer to the wiki
+
+  https://btrfs.wiki.kernel.org
+
+that maintains information about administration tasks, frequently asked
+questions, use cases, mount options, comprehensible changelogs, features,
+manual pages, source code repositories, contacts etc.