add post on "Reproducible builds, signing keys and binary repos"

As discussed in our weekly meeting recently, I made a little write-up on our experiences with RBs – and especially with the experiences when and after adding AllowedAPKSigningKeys for all apps in my repo. Thoughts, corrections etc. welcome – as well as merge & publish 😃

Notes:

•  URLs to other articles might need adjustments. Not sure about the syntax there: {% post_url .. %} or something?
•  URLs linking to our documentation as well (some are going directly to ../en/..). What about anchors in this case? As they are auto-generated from Markdown, they are language specific then.
•  Do links need to be inlined ([name](https://link/)) instead of being referenced ([name][1])?
•  apostrophs need to be unified. Which ones are preferred? "Normal ones" ('), or typographical (’)?