Patch 1 for Go

Combined pull request for two commits, first, sjakobi's additions for benchmarks and lint (PR https://github.com/exercism/docs/pull/27) and second, my note about optimization and a minor rewording.

The caution about optimization might seem not specific to Go but I think most languages don't have benchmarking test programs.  I wanted something in place as a guideline for nitpicking as well as for coding, to support arguments against over-zealous optimization.  (Probably because personally I'm afraid that sometimes I'm that zealot.)

The other issue i mentioned in another comment, about production code, I decided to hold off on.  I'm thinking it might be better brought up on an exercise by exercise basis, with comments in the test programs.