add note about exclusion of efficiency from transformer equation

uni/mmme/2051_electromechanical_devices/transformers.md

@@ -23,6 +23,8 @@ This coil has a current induced in it by induced magnetic field.
 $$\frac{i_s}{i_p} = \frac{v_p}{v_s} = \frac{n_p}{n_s} = \text{turns ratio}$$
 
 **Note that the current fraction is s over p, not p over s like the others.**
+Transformers have very high efficiency so will not be considered for this module and
+has been left out of the equation above.
 Derivation in lecture slides (pp. 12-15).
 
 Transformers are usually described as having a ratio $p:s$, where $p$ is the number of primary coils