Better installation instructions

[lumpidu]

There should be some better instructions for installation.

• hint on using git submodule update --init for the submodules
• a docker build line like docker build . -t tts-frontend:latest

Moreover, the submodules should reference the https://... URL's instead of the git@... -based URL's, otherwise it's immpossible to clone, if no SSL public key@github is registered for the user, e.g. if cloned from a remote server ...