added more hints what to consider when loading several plugins

now the documentation also explains via command line example how to load several plugins

doc/plugins.md

@@ -28,10 +28,17 @@ executable (e.g. use `chmod a+x plugin_name`)
 ## A day in the life of a plugin
 
 During startup of `lightningd` you can use the `--plugin=` option to
-register one or more plugins that should be started. `lightningd` will
-write JSON-RPC requests to the plugin's `stdin` and will read replies
-from its `stdout`. To initialize the plugin two RPC methods are
-required:
+register one or more plugins that should be started. In case you wish
+to start several plugins you have to use the `--plugin=` argument
+once for each plugin. An example call might look like: 
+
+```
+lightningd --plugin=/path/to/plugin1 --plugin=path/to/plugin2
+```
+
+`lightningd` will write JSON-RPC requests to the plugin's `stdin` and
+will read replies from its `stdout`. To initialize the plugin two RPC 
+methods are required:
 
  - `getmanifest` asks the plugin for command line options and JSON-RPC
    commands that should be passed through