Update contrib/initramfs/README.initramfs.markdown

Note that Dropbear supports ed25519 keys since version 2020.79.

See mkj/dropbear#91

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Michael Franzl <michael@franzl.name>
Closes #12715

contrib/initramfs/README.initramfs.markdown

@@ -78,7 +78,7 @@ To use this feature:
 1. Install the `dropbear-initramfs` package.  You may wish to uninstall the
    `cryptsetup-initramfs` package to avoid warnings.
 2. Add your SSH key(s) to `/etc/dropbear-initramfs/authorized_keys`.  Note
-   that Dropbear does not support ed25519 keys; use RSA (2048-bit or more)
-   instead.
+   that Dropbear does not support ed25519 keys before version 2020.79; 
+   in that case, use RSA (2048-bit or more) instead.
 3. Rebuild the initramfs with your keys: `update-initramfs -u`
 4. During the system boot, login via SSH and run: `zfsunlock`