Render READMEs by default on repo/tree pages (like GitHub)

Created by: dadlerj

Requested by https://app.hubspot.com/contacts/2762526/company/2931649618

As discussed a few weeks ago in Slack: https://sourcegraph.slack.com/archives/C0B2RU51Q/p1585148461000700

Is there any way to load/render the contents of README.md when browsing through the directory structure like github does?