Added docs on testmode!

docs/src/models/layers.md

@@ -66,6 +66,14 @@ LayerNorm
 GroupNorm
 ```
 
+### Testmode
+
+Many normalisation layers behave differently under training and inference (testing). By default, Flux will automatically determine when a layer evaluation is part of training or inference. Still, depending on your use case, it may be helpful to manually specify when these layers should be treated as being trained or not. For this, Flux provides `testmode!`. When called on a model (e.g. a layer or chain of layers), this function will place the model into the mode specified.
+
+```@docs
+testmode!
+```
+
 ## Cost Functions
 ```@docs
 mse