Idea: Add optional markdown block to top of reading group home page.

[jamie-reece]

We (University of London Press) will shortly be running an open review process using reading groups in Manifold. We've sent out guidance and will be inviting reviewers soon, but it occurred to me it might be useful to have a text block somewhere on the reading group homepage that gives owners the option to add some contextual info to reassure users they're in the right place.