add info about snapshot testing to style guide

Author: ArcAnya

docs/style guide.md

@@ -1,3 +1,15 @@
+For testing: 
+- try to use .toMatchSnapshot() when possible. Note that it will NOT be possible to make use of .toMatchSnapshot for the component as a whole if 'useContext' is used within the component.
+- if some of the child components of the tested component uses 'useContext' then you should use ShallowRenderer from 'react-test-renderer/shallow' as follows in order to limit the Snapshot to the component itself:
+```jsx 
+it('should render match DOM Snapshot', () => {
+    const shallow = new ShallowRenderer();
+    shallow.render(<MyComponent />);
+    const tree = shallow.getRenderOutput();
+    expect(tree).toMatchSnapshot();
+  });
+```
+
 When dispalying that could require ternaries, don't use ternaries unless its really simple.
 For example:
 ```jsx