Update README link to ketrew+coclobas instructions

[ryan-williams]

Currently points to https://github.com/hammerlab/epidisco/blob/master/docs, probably should at least point to https://github.com/hammerlab/epidisco/blob/master/docs/running-on-coclobas.md directly, and perhaps directly to https://github.com/hammerlab/secotrec if that's the new recommended doc.