Resolve "Problems running bilby_pipe with custom waveform model"

Closes #245 (closed)

docs/customisation.rst

@@ -12,7 +12,8 @@ and source model from some external package the following should be included in
     frequency-domain-source-model = my_package.submodule.custom_source_model
 
 In order to be compatible with the :code:`bilby_pipe` analysis scripts, custom classes should take the same arguments
-as their parent classes.
+as their parent classes. If you do not want to hard code all such arguments, you can use :code:`**kwargs` to capture
+(and ignore) additional arguments.
 An exception to this is when passing custom likelihood classes.
 In this case additional keyword arguments can be passed through the ini file, as below