Improve roadmap visibility

Currently, the roadmap can be found by accessing https://github.com/jquery/globalize/milestones and by clicking the "[read] more" link. Although, it's impossible (or at least very difficult) for someone to find it and it requires constant manual updates/maintenance.

We should find out a better solution: (a) to improve the access to this document and (b) to require less manual maintenance.

General ideas:
- [x] Benchmark other projects;
- [x] Link Roadmap in README.md's Devlopment section;
- [x] Have a "Current sprint" label to make it clear which Issues are in spotlight / team is currently working on.

Start by reading README.md's Development section and checking the GitHub milestones page, then review the roadmap access and manual-maintenance concerns described in the issue. Done means the roadmap is easier to find and requires less ongoing manual maintenance.

Written by the indexing model from the issue text.