Found an easy way to bring your Github fork up-to-date with the changes in the upstream master. No git command line required, takes about 10 seconds. The steps are:
1. In the Github fork (i.e. in YOUR repository), click "New Pull Request" 
2. You will see the message "Nothing to Compare", but click the "compare across forks" link.
3. Choose your fork on the left (the base repository), and the upstream source you want to sync with on the right (head repository). These will be the top 2 options in the drop-down.
4. Click Create Pull Request. Give the PR a name (like "Upstream merge") and click "Create pull request" to create it.
5. Congratulations, you have just created a pull request on your own fork! Now all you need to do is merge it by clicking "Merge pull request", and your fork is in sync with the upstream master.
Thanks to @bitfalls for the guide that led me to this solution, though the Github UI has changed a bit since then. So much easier than the "official" solution here: help.github.com/en/github/coll…
Forgot the link to @bitfalls's original guide, here it is: sitepoint.com/quick-tip-sync…
