Add explanation how to run two versions of Briar on same device

development-101.md

@@ -41,3 +41,28 @@ There are a few exceptions to this convention:
 Additionally we sometimes also use interfaces within packages.
 It can help establishing a clearer separation of concerns and easier testing
 when there's complex interaction between classes.
+
+## Running multiple versions of Briar on a single device
+
+Running two versions of Briar side by side on one device does not work out
+of the box. For example it is not possible to run the released version and
+the development version at the same time; only one of both will be online.
+The reason is that the both versions are trying to use the same hard-coded
+Tor ports to create their hidden service, which doesn't work. The solution
+is to use different ports on the development versions. To do so, change the
+following:
+
+In `bramble-api/src/main/java/org/briarproject/bramble/api/plugin/TorConstants.java`:
+
+    - int SOCKS_PORT = 59050;
+    - int CONTROL_PORT = 59051;
+    + int SOCKS_PORT = 59060;
+    + int CONTROL_PORT = 59061;
+
+In `bramble-core/src/main/resources/torrc`:
+
+    - ControlPort 59051
+    + ControlPort 59061
+      ...
+    - SocksPort 59050
+    + SocksPort 59060