[go/nucleotide-count] add mentoring notes

[bernot-dev]

Add mentoring notes for Nucleotide Count exercise.

[bernot-dev]

@IsaacG PTAL

[SleeplessByte]

@IsaacG can you give this a second look after the changes? @exercism/go maybe you want to give your opinion as well?

[IsaacG]

1. This still has multiple sentences per line which needs correcting.
2. A Go maintainer should look at this before it is merged. I'm not a Go maintainer.
3. This mostly looks like showcasing and walking through an example, not so much discussion points for mentors to raise.