Rooms need a way to indicate their spidering preferences

Analogous to robots.txt; would also be useful to indicate bridging preferences - i.e. 'even though this is a public room, please don't bridge it to other systems'

Start by reviewing the Matrix protocol specification's existing room-related sections and the discussion in issue #286. Determine how spidering and bridging preferences would be represented and where such a proposal belongs in the specification; done should be a resolved, accepted protocol design rather than an isolated implementation change.

Written by the indexing model from the issue text.