Small change to clarify where the TextSecure server URL should be changed to the staging value in local builds.
It looks like the Android variable has been updated from `TEXTSECURE_URL` to `SIGNAL_URL` (ae40715526 (diff-c197962302397baf3a4cc36463dce5eaR179)) and the iOS counterpart is named `textSecureServerURL`.
FREEBIE
Also make it accessible by providing a mode argument to the install
function. Previously developers could just edit the url but we no longer
have the address bar as an app window, so now they must close the
default installer and run the following from the background page
console: `extension.install('standalone')`.
In the production build, this should result in an error since it is not
supported / the register page is not included there.
Move the development setup instructions to CONTRIBUTING.md and add
some guidelines for pull requests and such.
README now contains crypto export notice, license, etc... Borrowed
heavily from the Signal-iOS readme.
