Link docker instructions from readme

[patcon]

So glad to see that docker is documented really well! Wondering if it would make more sense to be more DRY in the docs, and simply link to the instructions in slackarachive-docker rather than reproducing them in the slackarchive repo README :)

[yanokwa]

I'd go the other way and put the instructions in this repo's readme and put links in the various sub-projects back to this repo. I suggest this option because people seem to link here and file issues here.