Update moderator guidelines

### Summary

In our co-working session we discussed guidelines for moderators to
1) highlight which parts of a submitted story contravene the code of conduct
2) share the parts of the code of conduct which are contravened
3) include these as comments in the moderation page, to be shared with the user who submits the story

This will allow a non-personal, rule-based clarification of the problem and make it easier for story-submitters to understand the decision and revise their story if they wish,

### What needs to be done?

- [ ] Georgia to write guidelines and share them on this issue
- [ ] Guidelines to be added to the platform for users logged in as moderators