Update README (add support for busybox and debian)

[pschaub]

No description provided.

[MrOggy85]

I would suggest to add this to the Alpine Linux section:

RUN apk update && \
  apk add ca-certificates wget && \
  update-ca-certificates && \
  apk add --no-cache openssl

since I needed this to be able to get the source when using the node alpine linux docker image (version 8.11.4-alpine). I cannot say if it's a general thing though...
Reference discussion: Yelp/dumb-init#73

[jwilder]

Closing — this has been open for 7+ years and the README has drifted significantly since then. Not sure the busybox instructions are still relevant with current install methods. Thanks for the contribution!