Add hints to some exercises

[abo64]

Most exercises have their own favorable solutions and characteristic pitfalls.
To give people some guidance and avoid writing repetitive or similar code comments again and again
we should add a HINTS.md file to any exercise where it makes sense.

Especially the first couple of exercises should have such a file to assist beginners.
Another case is exercises dealing with special features of the language like Option or Either.

Here is a first short list of exercises that should contain hints. Please add more if you feel the need.
And add the checkmark to the respective exercise if you have added the hints.

• hello-world
• hamming
• nucleotide-count
• roman-numerals
• bank-account
• phone-number
• robot-name
• leap
• gigasecond
• wordy
• parallel-letter-frequency
• sgf-parsing
• zipper
• forth
• lens-person

[ErikSchierboom]

Great idea. I would also suggest to include hints for the text-parsing exercises to Scala's parser combinator feature.

For the Option and Either hints, we could link to the Neophyte's Guide to Scala, which has an excellent explanation of those topics.

[abo64]

@ErikSchierboom I am working on Option and Either hints (also on hello-world), and I also love "Neophyte's Guide to Scala"! ;-) Hope to have the PR ready soon.
And yes, text-parsing is definitely another candidate for hints. All exercises that go way beyond standard Scala, especially when a third party library is involved (another such example is the lenses exercise).

@ErikSchierboom @ricemery First I intended to add the whole list of exercises to this issue so that we can checkmark those containing hints, but decided against it because I thought we will have hints only for a couple but not all of them. Or perhaps add such a list, but only for those exercises that need hints?
Another question: Is it ok to create a PR to add hints to more than just one exercise? Or is it better to have one PR per exercise?

[ErikSchierboom]

I don't think we should do this for all exercises, but only for those where it makes sense. Also, in general, the smaller the PR the better, so my preference would be to create one PR per exercise.

Creating a list for all exercises that need hints might be wise, as it allows us to keep track of progress.

[abo64]

thx @ErikSchierboom
I will add a such a list, and you guys feel free to add/modify it.
And let's not forget to keep it up to date.

EDIT: I added the list.