Remove Docker Swarm requirement from docs

[e13h]

Resolves #143

[tangowithfoxtrot]

Unless Docker has enabled secret usage in non-Swarm deployments (I would love if they did), I believe it makes sense to list Docker Swarm as a requirement in this particular doc because it's specifically for docker secrets.

Using env files is a different use-case than secrets, but perhaps it wouldn't hurt to provide examples for non-Swarm deployments somewhere else (like here) though?

I'm not the maintainer though, so they may disagree.