[archiso] Update README.build

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>

docs/README.build

@@ -42,9 +42,11 @@ INDEX
 *** Image types generated by mkarchiso.
 
 * image-name.sfs     SquashFS image with all files directly on it.
+                     [read-only, no dm-snapshot is used]
 * image-name.fs.sfs  SquashFS with only one file inside (image-name.fs),
                      which is an image of some type of filesystem
-                     (ext4, ext3, ext2, xfs), all files reside on it.
+                     (ext4, ext3, ext2, xfs, btrfs), all files reside on it.
+                     [read-write, via COW image with dm-snapshot]
 
 
 *** File format for aitab.
@@ -107,6 +109,6 @@ used to build official images with much more things.
   # make -C archiso install
 
 * Build them!
-  # /usr/share/archiso/configs/releng/build.sh build all
+  # /usr/share/archiso/configs/releng/build.sh
 
-Note: See build.sh -h for more options.
+Note: See build.sh -h for more options. This only runs on x86_64.