First stab at contributing guide (#1427)

* First stab at contributing guide

* files(docs) - create docs folder, move overview doc there

docs/overview.md

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+## Brief Overview
+
+TL;DR: Code = React + TS, heavy use of useCallback / memo for performance.
+
+NB: all filenames given without path unless there are two identical filenames.
+
+### Architecture
+
+We use a redux-like architecture for most editor-wide settings, with the main exception being the code<>design loop. The state is in `editor-state.ts`. State contains both transient and non-transient parts for undo purposes. The following is a quick guided tour to add a setting, and the actions to change it.
+
+In your component, you may want to call an action to change setting. To do that, call dispatch with EditorActions (if you search for this, you'll find lots of examples & the required imports).
+
+```
+const toggleAdditionalControls = React.useCallback(() => {
+    dispatch([EditorActions.toggleInterfaceDesignerAdditionalControls()])
+  }, [dispatch])
+```
+
+The action lives in `action-creators.ts` and returns the action - it could also return other fields for payload.
+
+```
+export function toggleInterfaceDesignerAdditionalControls(): ToggleInterfaceDesignerAdditionalControls {
+  return {
+    action: 'TOGGLE_INTERFACEDESIGNER_ADDITIONAL_CONTROLS',
+  }
+}
+```
+
+In `action-utils.ts` there's a list of _transient_ actions, of which this is one. If you're creating a transient action, add it there!
+
+In `editor-update.ts` we map `TOGGLE_INTERFACEDESIGNER_ADDITIONAL_CONTROLS` to an actual function, `return UPDATE_FNS.TOGGLE_INTERFACEDESIGNER_ADDITIONAL_CONTROLS(action, state)` specifically.
+
+In `actions.ts` you'll find the actual action. It returns the editor, with only the changing values overwritten. The action here doesn't have a payload (it just toggles the `additionalControls`, as you can see here. If there were a payload, it'd be part of the `action` object.
+
+```
+TOGGLE_INTERFACEDESIGNER_ADDITIONAL_CONTROLS: (
+    action: ToggleInterfaceDesignerAdditionalControls,
+    editor: EditorModel,
+  ): EditorModel => {
+    return {
+      ...editor,
+      interfaceDesigner: {
+        ...editor.interfaceDesigner,
+        additionalControls: !editor.interfaceDesigner.additionalControls,
+      },
+    }
+  },
+```
+
+If you want to add a new property to the editor state, the safest way to do that is to add it to the EditorState interface, and then work your way through the compile errors.
+
+### Layout, Theme, Icons, Standard Components
+
+Most of these live in `uuiui`. We mainly use flexbox and css grid for layouts. Most of our code relies on `InspectorRow` (css grid), FlexRow, and FlexColumn.
+Our theme lives in `theme.js` for the overall editor (excluding VSCode).
+
+### Code Editor
+
+We embed VSCode (not Monaco), with an extension to keep it in sync with the design tool and vice versa.