Out and in docs

These docs aim to (minimalistically) describe how remotestorage.js works on the *out*side (e.g. how an app developer can use it), as well as on the *in*side (i.e. how the code works)
just realized we obviously haven't release these changes on npm yet, since we haven't done a release since they happened. Documenting the current status quo now, will try to tag a release on saturday, after that @silverbucket or I or someone else can finish https://github.com/remotestorage/remotestorage.js/issues/768 (once it's actually working with what is on npm)