Add a guide for writing a simple recurrent network #511

Hi!
Axon beginner here.

I struggle to figure out how to write a very simple RNN network. Basically I want to rewrite this pytorch example from a tutorial.

However, the Axon API makes it a bit convoluted to create networks that "scan" or "unroll" the input. After some digging I realized that I need to create something similar to lstm_cell and lstm, but these APIs are not well documented (what do dynamic_unroll arguments mean?). I am also not sure how to handle parameters in that case so the training mechanism (Axon.Loop.trainer with standard optimizer and loss functions) can do it's job.