Add Runtastic download and convert to gpx method in README

[yihong0618]

Add another sport app which can export tracks in README

[c-harding]

What format does Runtastic output? Is it possible to accept it directly?

There are also a few grammatical errors I’ll fix later today.

[yihong0618]

Thank you very much.
@xsanda many json files are in different folders such as the picture below

[erik]

Closing this out as it's been quite a while now. Feel free to reopen if it's still relevant

[yihong0618]

@erik OK
and I made a project using mapbox support the gpx too, and support most running app
If you interest you can find here.
https://github.com/yihong0618/running_page

and thank you for your great repo.