[Roman Numerals] add practice exercise (v2)

[colinleach]

Let's try this again, on a new day.

For the week of [02-06]. The example.R doesn't exactly showcase R's best features, but it's the best I've come up with so far.

[colinleach]

I have some draft approaches ready to PR, but I'll wait until this exercise is merged so I have a base for adding the .approaches directory.

I think this link is to the new branch on my fork.

[jonmcalder]

Sorry for the slow turnaround @colinleach! Haven't forgotten about this and will review as soon as possible.

[jonmcalder]

Agree that the example solution might not be the most elegant or idiomatic R code (appending within a loop), but it gets the job done and no reason to hold back on adding this exercise (it's late and I'm not in the right headspace to think of alternatives right now)

I removed myself from authors (you've been generously including me as co-author in many of your PR's but that's only really necessary if the review substantially changes or adds to the PR)

[colinleach]

Once the exercise is merged I can rebase the approaches and PR them. There might be better solutions in there, that can be copied across later.

[jonmcalder]

I've just skimmed through the approaches in your branch which look good - thanks. I'm sure we can get those merged in over the weekend ahead of next week's #48in24