add getting started video 12: semantic versioning

Here's the 12th Getting Started video. If you'd like to review and comment, please feel free to!

https://www.youtube.com/watch?v=e4bpo2KUvm0

Before merge, these must be done:
- [ ] @rod11 reviews
- [x] @iarna reviews
- [ ] Lin smooths out vocals
- [ ] Lin adds text