Update CONTRIBUTING.md - Adding info about issues within a milestone

What does this MR do?

Adds copy regarding issues within the current milestone to the contribution guide.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

cc @dzaporozhets @awhildy

@tauriedavis looks good, thanks

Mentioned in commit 0aca9472

Status changed to merged

Mentioned in merge request !7507 (closed)

Milestone changed to %8.15