Convert the check script to a script that sets the version to match
whatever Riot is being packaged.
Add a README because the two ways of interacting with versions are
starting to get complex to reason about.
Also add a docker build script.
* Replace the windows signing script with certificateSubjectName
option which finds the right certificate from the smart card
* Remove the windows signing script as electron-builder does it
out-of-the-box on windows
* Remove the cert file which isn't necessary with native signing
* Remove install:electron - it ran install-app-deps which makes sure
native deps are installed but the native deps we use don't work
with electron-builder anyway (they don't use binding.gyp) so this
does absolutely nothing at all.
* Rename build:electron to build (there is only one thing to build
now).
