add note on MIMO

Author: baggepinnen

README.md

@@ -71,6 +71,8 @@ julia> latextf(G)
 ## Code generation
 The function `code = SymbolicControlSystems.ccode(G::LTISystem)` returns a string with C-code for filtering of a signal through the linear system `G`. All symbolic variables present in `G` will be expected as inputs to the generated function. The transfer-function state is handled by the C concept of `static` variables, i.e., a variable that remembers it's value since the last function invocation. The signature of the generated function `transfer_function` expects all input arguments in alphabetical order, except for the input `u` which always comes first.
 
+Code generation for systems with multiple inputs and outputs (MIMO) is only handled for statespace systems, call `ss(G)` to convert a transfer function to a statespace system.
+
 
 A usage example follows
 ```julia