Update RFC-00xx-automatic-implicit-differentiation.md

Author: 0x00b1

RFC-00xx-automatic-implicit-differentiation.md

@@ -315,6 +315,12 @@ $$\min_{x} \frac{1}{2} ||\textbf{r}(x, \theta)||^2,$$
 
 where $\textbf{r}$ is a residual function, $x$ are the parameters with respect to which the function is minimized, and $\theta$ are optional additional arguments.
 
+The update equation is solved for every iteration to find the update to the parameters:
+
+$$\mathbf{J} \mathbf{J^T} h_{gn} = - \mathbf{J^T} \mathbf{r}$$
+
+where \mathbf{J} is the Jacobian of the residual function w.r.t. parameters.
+
 ##### Gauss–Newton
 
 ```Python