control: README note on limited JSON support

Add a note to the README that the fan control application's JSON
support is limited to only setting fans within a zone to the configured
`full_speed`.

Change-Id: Ifd4e4515847c8766ad224ebaf27853ca4d0518ed
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
1 file changed