Add Best Practices Section for Random Tests

[DonaldKellett]

This pull request adds a section in the markdown file regarding writing random test cases for beginners on Codewars. This extra section is on Best Practices and explains to the reader how to further prevent cheats and/or unexpected results when writing random tests. The author of this pull request has spent over a year on the Codewars platform and has authored a total of 82 Kata at the time of writing, over 75% of which were highly successful so hopefully this addition to the documentation can further guide beginners on how to write excellent random tests on the first try. If needs be, I could instead move the new content that I just added into another file named something like random-tests-for-advanced-kata-authors.md instead of adding the content directly to the existing Markdown file. I’ve spent the past hour adding the content mentioned so it would be great if it was accepted and merged into the official Repo, thanks 😄