Setup instructions leave out step

[Venryx]

The setup instructions for starting local development is missing a step: https://github.com/mobxjs/mobx-devtools/blob/master/HACKING.md#chrome-extension

Instructions listed:

1. run git submodule update --init
2. run yarn
3. run yarn start:chrome
4. Go to chrome://extensions, check "developer mode", and click "Load unpacked extension", and select directory lib/chrome

However, step 4 fails, saying it can't find the "icons/toolbar-chrome.png" file.

To fix, I just needed to copy the "html" and "icons" folders from "src/shells/webextension" into the "lib/chrome" folder. However, it would be good to add this to the readme.

[Venryx]

Nevermind, it appears to be an issue because I'm on the Windows platform, so this loader test fails for me: https://github.com/mobxjs/mobx-devtools/blob/master/src/shells/webextension/webpack.config.js#L37

When I change the test to allow for path-seperators of either / or \:

test: /icons[/\\].*\.(png|svg)$/,

Then the files in the icons folder are outputted correctly to the "lib/chrome" folder.

Should this change be added to the webpack config, for developers on Windows?