Add testing section to CONTRIBUTING.md.
Add testing section to CONTRIBUTING.md.

Request help with testing for:
* Tablet devices
* Apple Mac compatibility
* Flights that cross midnight (UTC).
GlidingWeb/IGCWebView,brisbori/jsigc,brisbori/jsigc,alistairmgreen/jsigc,GlidingWeb/IGCWebView,GlidingWeb/IGCWebView,alistairmgreen/jsigc