Add mentoring notes for core exercises

To help our mentors, we should add mentoring notes for each core exercise. These are the core exercises (excluding `hello-world` which is automatically approved), with a check if there already are mentoring notes for that exercise:

- [x] two-fer
- [x] leap
- [x] gigasecond
- [ ] series
- [x] space-age
- [ ] high-scores
- [x] hamming
- [ ] nucleotide-count
- [ ] robot-name
- [x] allergies
- [ ] grade-school
- [ ] rotational-cipher
- [x] clock
- [x] bob
- [ ] circular-buffer
- [ ] bracket-push
- [ ] spiral-matrix
- [ ] tournament
- [ ] variable-length-quantity
- [ ] dominoes
- [ ] forth

The mentoring notes themselves can be submitted to: https://github.com/exercism/website-copy/tree/master/tracks/csharp/exercises