insights: document supported scale

Created by: coury-clark

Code Insights behaves in a way that isn't always obvious with respect to customer scale. This issue is to provide some clearer guidelines on what kind of scale we currently support, what kind of scale we will likely support in the near future. Some criteria for scale:

1. number of repos
2. corpus size
3. number of users
4. open vs restrictive (repo permissions)
5. sub-repo permissions