[testpilot] Updated instructions to test dev server

[Francesco Lodolo [:flod]]

Hi,
I've discovered this morning that the dev server for Test Pilot doesn't
work anymore "out of the box"
https://testpilot.dev.mozaws.net/

You need to open about:config in Firefox and:
* Create a key ' testpilot.env' (type: string), or modify it if it
already exists, and set it to 'dev' (without quotes).
* Create a key 'extensions.webapi.testing' (type: boolean) and set it to
True.

At this point the URL should open correctly.

There's already an issue filed for the date in the newsfeed not being
properly localized.
https://github.com/mozilla/testpilot/issues/2583

Talking about TestPilot: last week a few strings landed, and they were
changed significantly after a few hours.
https://github.com/mozilla/testpilot/issues/2574

It's really hard to tell how many locales were impacted by this change,
and I want to ask for new string IDs to make sure it's fixed in all
languages. In the meantime, you can search for 'Eolwarning' in Pontoon
(in All Resources or directly in experiments.ftl), and check that your
translation matches the English text.

This kind of problems should not happen anymore in the future, when all
string changes should land in a different branch after review, and only
then exposed for localization via Pontoon.

Francesco