DrumVariables issue reference

README.md

@@ -133,6 +133,7 @@ where it improves legibility. Consult the source files for more information.
 
 <a name="drumvars"></a>
 #### `DrumVariables` (drum_variables.hpp), contains data members characterizing the dynamic state of a drum
+- Respect the constraints given by [this issue](https://gitlab.phys.ethz.ch/engelerp/rbcomb-simulation/issues/4).
 
 <a name="drum"></a>
 #### `Drum` (drum.hpp), represents a single drum top resonator