readme: add description of exclusions

Author: carlaKC

README.md

@@ -104,7 +104,8 @@ To run the simulator with random activity generation, you just need to
 provide a set of nodes and the simulator will generate activity based 
 on the topology of the underlying graph. Note that payments will only 
 be sent between the `nodes` that are provided so that liquidity does 
-not "drain" from the simulation.
+not "drain" from the simulation. Every node specified will be eligible
+to send and receive payments when running with random activity.
 
 ```
 {
@@ -348,6 +349,27 @@ Note that you need to provide forwarding policies in each direction,
 because each participant in the channel sets their own forwarding 
 policy and restrictions on their counterparty. 
 
+
+### Random Activity Exclusions
+
+When running with random activity (implied by the absence of `activity`),
+every node listed in `sim_network` will be eligible to send and receive
+payments. If you'd like to specify nodes in `sim_network` that do not
+send/receive payments, you can include them in an `exclude` list:
+
+```
+{
+  "sim_network": [ ... ],
+  "exclude": [
+    "020a30431ce58843eedf8051214dbfadb65b107cc598b8277f14bb9b33c9cd026f"
+  ]
+}
+```
+
+This is required for the case where you want to specify a channel that
+will be used in the simulated network that is just responsible for 
+forwarding payments, but does not send or receive any payments itself.
+
 ### Inclusions and Limitations
 
 The simulator will execute payments on the mocked out network as it