Link where to find unbranded builds of Release and Beta in DEV.md

[biancadanforth]

Since most Shield studies are intended for the Release channel, and some may require pre-release testing, it would be good for developers to be able to test unsigned legacy addon studies (a WebExtension Experiment and a bootstrapped addon are both in this category) locally in those versions of Firefox.

Unfortunately, as of Firefox 57, unsigned legacy addons are not permitted in regular Release or Beta Firefox, so we have to use unbranded builds for testing.

We should note in DEV.md where developers can find these unbranded builds, and how they can set their path environment variable FIREFOX_BINARY (I use ~/.bash_profile) to an unbranded build path to work with npm run firefox.