ad485x_fmcz: Update doc

Author: StancaPop

docs/projects/ad485x_fmcz/index.rst

@@ -193,6 +193,13 @@ To build the LVDS version of the interface, run:
    $cd hdl/projects/ad485x_fmc/zed
    $make LVDS_CMOS_N=1
 
+The default LVDS configuration is for 4 wire SPI. To use the 3 wire SPI, run:
+
+ .. shell:: bash
+ 
+   $cd hdl/projects/ad485x_fmc/zed
+   $make LVDS_CMOS_N=1 THREE_W_SPI=1
+
 A more comprehensive build guide can be found in the :ref:`build_hdl` user
 guide.