API : Add a header for rate limiting

[Riduidel]

I'm working on a project to get download infos for various platforms (https://github.com/Riduidel/aadarchi-technology-detector).
For this project, I scan pypistats result for ~ 300 packages.
It seems like it's enough for my ip to get rate limited. And that's perfectly ok.
However, is it possible to have an http header added in order for me to know how much request I can make, how long I'll have to wait, and so on?

[hugovk]

If it helps to know, the limit is 5 per second and 30 per minute:

pypistats.org/pypistats/run.py

Line 23 in 70a10f9

limiter = Limiter(app, key_func=get_remote_address, application_limits=["5 per second", "30 per minute"])