From his website, "In my spare time, I contribute to the open source Fluentd Project's documentation." In fact, you probably don't even need to understand Git, since pretty much everything can be done via the browser now. Clicking on the "Edit" button on README/documentation pages automatically forks the repo and makes it trivial to submit patches.

Probably the biggest contribution a non-coder could do is bug-triaging, which means: Look into newly reported issues and try to reproduce them and check for duplicate issues.

In the best case, you can simply reproduce it. Then say so in a comment to the issue. However more probable, you have to ask the reporter for more details or try different versions and environments.