btrfs-progs: build: note minimal version for zoned support

The configure phase can detect the following:

- no /usr/include/blkzoned.h - no zoned support possible
- usable /usr/include/blkzoned.h - on by default
- present /usr/include/blkzoned.h but unusable due to missing struct
  members or ioctl defintion

Case 3 could be confusing, so document the requirements.

Issue: #370
Signed-off-by: David Sterba <dsterba@suse.com>

INSTALL

@@ -22,6 +22,9 @@ dependencies are not desired.
 - libsodium
 - libkcapi
 
+For zoned device support, the system headers installed in /usr/include/linux
+must be 5.10 or newer.
+
 Generating documentation:
 
 - asciidoc - text document format tool