Track changes mode

First of all: I really love hackmd! It's a great tool and my organization uses it on a day to day basis!

We mainly use it as a fast and easy way to write down some structured ideas or to take minutes of meetings. For the review of minutes it would be awesome to have something like a tracking mode (cf. GoogleDocs, Word or other editors) where e.g. the initial author of a document can freeze its state and all changes have to be accepted by him afterwards.

Knowing about the revision view I hope it would be possible to include this additional feature and I hope it would be appreciated by others as well.