Added note about functional tests ADDON_ZIP env var

docs/DEV.md

@@ -145,6 +145,8 @@ ln -s Firefox\ Nightly.app FirefoxNightly.app
 
 ### Functional tests
 
+(To run the functional tests, package.json has to be updated to reflect the built add-on zip filename the declaration of `npm run test:func` - this is silly and there is an issue about it here: https://github.com/mozilla/shield-studies-addon-template/issues/75 - but we are stuck with having to manually update it for the time being.)
+
 ```shell
 npm run test:func
 ```