Documentation: update btrfs list of features and link to readthedocs.io

The btrfs documentation in kernel is only meant as a starting point, so
update the list of features and add link to btrfs.readthedocs.io page
that is most up-to-date. The wiki is still used but information is
migrated from there.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/filesystems/btrfs.rst

@@ -19,13 +19,23 @@ The main Btrfs features include:
     * Subvolumes (separate internal filesystem roots)
     * Object level mirroring and striping
     * Checksums on data and metadata (multiple algorithms available)
-    * Compression
+    * Compression (multiple algorithms available)
+    * Reflink, deduplication
+    * Scrub (on-line checksum verification)
+    * Hierarchical quota groups (subvolume and snapshot support)
     * Integrated multiple device support, with several raid algorithms
     * Offline filesystem check
-    * Efficient incremental backup and FS mirroring
+    * Efficient incremental backup and FS mirroring (send/receive)
+    * Trim/discard
     * Online filesystem defragmentation
+    * Swapfile support
+    * Zoned mode
+    * Read/write metadata verification
+    * Online resize (shrink, grow)
 
-For more information please refer to the wiki
+For more information please refer to the documentation site or wiki
+
+  https://btrfs.readthedocs.io
 
   https://btrfs.wiki.kernel.org