Added mean_S0samples description

doc/fdt_bedpostx.html

@@ -62,6 +62,8 @@ anisotropy  </li>
 <ul><li>Note that in each voxel, fibres are ordered according to a decreasing mean f-value</li></ul>
 <li><b>mean_dsamples</b> -  3D Volume - Mean of
 distribution on diffusivity d</li> 
+<li><b>mean_S0samples</b> -  3D Volume - Mean of
+distribution on S0 intensity</li> 
 <li><b>dyads&#60;i&#62;</b> - Mean of PDD distribution in vector form.
  Note that this file can be loaded into fslview for easy <a href="fdt_display.html">viewing of diffusion directions</a></li>
 <li><b>dyads&#60;i&#62;_dispersion</b> - 3D Volume - Uncertainty on