update howto

how_to.mdwn

@@ -42,6 +42,8 @@ OP<sub>obs</sub> = G × β + ε
 where G is the contribution of each factor given by the PMF, OP is the observed OP
 (DTT or AA), β is the *intrinsic OP* and ε the residual term.
 
+  * **Obs. vs model**: scatter plot of the observation against the model OP
+    reconstructed by the MLR ;
   * **Intrinsic OP**:  display the intrinsic OP (i.e. coefficient of the MLR)
     for each PMF factor at each site, together with their uncertainties;
   * **OP contribution (all)**: give the overall PMF factor contribution to the