Add copy about ordering exercises to exercises.md -- v2

[jbergenson]

Reset the branch a bit too far back when consolidating commits on the last PR. Thought it would be cleaner to create a new PR with changes suggested in the comments in #129.

Quoting #129:

Add copy about ordering exercises in exercises.md

Original issue: #116

[petertseng]

This comment does not require any response or change to this PR, it is FYI for future reviewers only:

I think it is correct that this PR does not specify a keyword that closes #116 ; it would need to explain the significance of the ordering in order to do so (core exercises are presented in the order they appear in the array; non-core exercises' ordering has no Exercism-affecting significance so any ordering among non-core exercises is solely for the benefit of the humans). Therefore, #116 should still remain open even after this PR is merged. Thank you.