Add product values to onboarding reading list and task list

We originally wanted the Hypothesis Product Values document to appear in two places in the onboarding process:

- [ ] 1. It should be in the [pre-onboarding reading list](https://github.com/hypothesis/onboarding/issues/19)
- [ ] 2. There should be a task list item in the **Making simple contributions** section of the onboarding task list for the engineer to (re-)read the Product Values before making their first contribution

We can't reference the Product Values document in either of those two locations right now because, as far as I know, there is no Product Values document.

This issue is to add links to the Product Values document in the two locations once we have the document