Add FAQ about rate-based networks (#531)

docs/faq/question_03.md

@@ -6,7 +6,7 @@
 
 - single-compartment (point neuron) Hodgkin-Huxley models
 - multi-compartment Hodgkin-Huxley models
-- rate-based neuron models
+- rate-based neuron models (tutorial [here](https://jaxley.readthedocs.io/en/latest/faq/question_04.html))
 
 For all of these models, `Jaxley` is flexible and accurate. For example, it can flexibly [add new channel models](https://jaxleyverse.github.io/jaxley/tutorial/05_channel_and_synapse_models/), use [different kinds of synapses (conductance-based, tanh, ...)](https://github.com/jaxleyverse/jaxley/tree/main/jaxley/synapses), and it can [insert different kinds of channels in different branches](https://jaxleyverse.github.io/jaxley/tutorial/01_morph_neurons/) (or compartments) within single cells. Like `NEURON`, `Jaxley` implements a backward-Euler solver for stable numerical solution of multi-compartment neurons.
 

docs/faq/question_04.md

@@ -0,0 +1,52 @@
+# How can I implement rate-based neuron models in Jaxley?
+
+In this FAQ, we explain how one can implement rate-based neuron models of the form:
+$$
+\tau \frac{dV}{dt} = -V + \sum w_{\text{syn}} \phi(V_{\text{pre}})
+$$
+Here, $\phi$ is a nonlinearity such as a `TanH` or a `ReLU`.
+
+To implement this in `Jaxley`, we first have to set up a network consisting of 
+point-neurons:
+```python
+import jaxley as jx
+
+num_cells = 100
+cell = jx.Cell()  # Create a point-neuron.
+net = jx.Network([cell for _ in range(num_cells)])
+```
+
+Next, we have to equip the neurons with a `Leak` so as to model:
+$C \cdot dV/dt = -V$
+
+```python
+from jaxley.channels import Leak
+
+net.insert(Leak())
+net.set("Leak_eLeak", 0.0)  # Center the dynamics around zero.
+net.set("Leak_gLeak", 1.0)  # We will deal with the time-constant later.
+```
+
+Next, we have to connect the cells with `Tanh` synapses:
+```python
+from jaxley.connect import fully_connect
+from jaxley.synapses import TanhRateSynapse
+
+fully_connect(net.cell("all"), net.cell("all"), TanhRateSynapse())
+```
+
+Lastly, what rate-based neuron models call the time constant is called the `capacitance`
+in `Jaxley`:
+```python
+net.set("capacitance", 2.0)  # Default is 1.0.
+```
+
+That's it! As always, you can inspect your network by looking at `net.nodes` and
+`net.edges`.
+
+Equipped with this network, you can check out the 
+[tutorial on how to simulate network models in Jaxley](https://jaxley.readthedocs.io/en/latest/tutorials/02_small_network.html).
+You can also check out the
+[API reference on different connect() methods](https://jaxley.readthedocs.io/en/latest/reference/jaxley.connect.html)
+(e.g. `sparse_connect()`) or the
+[tutorial on customizing synaptic parameters](https://jaxley.readthedocs.io/en/latest/tutorials/09_advanced_indexing.html).