Display Dates for each Section

[IanVS]

Because this is going to be a "living" book and the frameworks change so fast, it's going to be important to display the date each section was written/published. This way people will be able to tell at a glance if it is possibly outdated or new information. Maybe the git commit date can be used?

Another feature that this would enable is a sorting or filtering of the book by date. Imagine a user reads the whole book, and then two months later checks back in to see if anything has changed. It would be pretty handy to be able to specify "show me anything new from the last 2 months" and see a list of the sections which have been added or updated. This likely can't be done on GitBook, but should be possible if another site/app is created for this repository.