Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -366,7 +366,7 @@ For many typical features one would want to add that involve changing some aspec
     **Note about when to use global variables in view code:**
     In general, you should not access global variables in view code, e.g., don't do this: `var helix = app.state.design.helices[0]`. The whole idea that makes React work is that each component produces its HTML as a pure function of its props (and its React state, which is used in only a few rare instances in scadnano). The algorithm used to update the view efficiently makes this assumption. Thus, if you are tempted to access a global variable, most likely what needs to happen is that more props need to be added to the component, getting that information from the the ancestor components, which ultimately get them from a Redux connected component that can access the global state directly.
 
-    Note that this is a rule about code that **renders** the view (i.e., anything called by the `render` method of a component). But it does not apply to *event handling code* that the view declares (but does not call directly). Event handlers are functions that execute in response to (typically) user interaction with the view *after* it has already been drawn. For example, you'll see lines like this `..onClick = on_mouse_click` where `on_mouse_click` is a function taking an event type as input and indicating how to respond. It is fine for event handling code to access the global app state variable `app.state`. In fact, this is preferred over passing the information it needs as props. The reason is that if only event-handling code needs that information, but not the view code, then whenever the information changes, the view will re-render, causing slowdown, even though the appearance of the component didn't change. (All that changed was what will happen eventually when the event happens that triggers the event handler.) Read more about this issue here: https://github.com/UC-Davis-molecular-computing/scadnano/issues/191#issuecomment-659729707
+    Note that this is a rule about code that **renders** the view (i.e., anything called by the `render` method of a component). But it does not apply to *event handling code* that the rendering code declares (but does not call directly). Event handlers are functions that execute in response to (typically) user interaction with the view *after* it has already been rendered. For example, you'll see lines like this `..onClick = on_mouse_click` where `on_mouse_click` is a function taking an event type as input and indicating how to respond. It is fine for event handling code to access the global app state variable `app.state`. In fact, this is preferred over passing the information it needs as props. The reason is that if only event-handling code needs that information, but not the view code, then whenever the information changes, the view will re-render, causing slowdown, even though the appearance of the component didn't change. (All that changed was what will happen eventually when the event happens that triggers the event handler.) Read more about this issue here: https://github.com/UC-Davis-molecular-computing/scadnano/issues/191#issuecomment-659729707
 
 7. **if necessary, add middleware:** 
     The above description of state, view, and reducers in React/Redux describes an ideal situation in which every reducer and every view is a pure function, dependent only on the state (and also action for reducers), taking no other data as input and affecting no other part of the memory (i.e., having no *side-effects*). Of course, sometimes programs need to break this, having side-effects such as saving files, or taking other inputs not available in the React props at the time the Action was created (for example, some parts of the view may not have enough data to check whether an action is legal to dispatch).