Provide access to user research

At the [F2F meeting in London](https://github.com/ampproject/meta-ac/blob/master/meetings/2019-03-f2f.md#is-amp-a-product-or-an-open-source-project), it was brought up that it's very difficult for the TSC or the AC to make decisions without access to existing but non-public user research.

@edent and @LJWatson volunteered to draft an _opinion_ on the topic to submit to the AC for further discussion.