Remove og:image with status.im URL

And make it configurable, albeit only at buuld time, as hopefully
explained in the README.

README.md

@@ -149,6 +149,17 @@ For a good example, see https://riot.im/develop/config.json
    with referral tracking; please ignore it.
 1. `welcomeUserId`: the user ID of a bot to invite whenever users register that can give them a tour
 
+
+Note that `index.html` also has an og:image meta tag that is set to an image
+hosted on riot.im. This is the image used if links to your copy of Riot
+appear in some websites like Facebook, and indeed Riot itself. This has to be
+static in the HTML and an absolute URL (and HTTP rather than HTTPS), so it's
+not possible for this to be an option in config.json. If you'd like to change
+it, you can build Riot as above, but run
+`RIOT_OG_IMAGE_URL="http://example.com/logo.png" npm run build`.
+Alternatively, you can edit the `og:image` meta tag in `index.html` directly
+each time you download a new version of Riot.
+
 Running as a Desktop app
 ========================