Add guideline about "Make hacks obvious"

[hteumeuleu]

On Slack, @JayOram says:

We tend to work on the rule - if I was to show this to a junior email dev, or a web developer and ask them to edit it - is there something they could miss.

And I love this! I've long done this without formalizing it this way. Here are different forms this can take:

Add a prefix to a class name

<div class="gmail-blend-difference"></div>

Add a distinct <style> tag with a specific id

<style id="css-laposte">
…
</style>