Add transfer learning section to readme.

README.md

@@ -51,6 +51,14 @@ match exactly. Feel free to use it via `pretrained_resnetd` (should be fine for
 transfer learning). You must install fast.ai yourself
 ([instructions](https://docs.fast.ai/)) to use this function.
 
+### Transfer Learning
+
+The `slice_model` function (found in in
+[`common.py`](https://github.com/n2cholas/jax-resnet/blob/main/jax_resnet/common.py))
+allows you to extract a subsection of the model and variables dict (e.g.
+everything but the Dense layer, so you can add your own classification head).
+Check out that docstring for more information.
+
 ## References
 
 - [Deep Residual Learning for Image Recognition. Kaiming He, Xiangyu Zhang,