Add documentation to use GlitchTip with Cypht Webmail

[marclaporte]

Let's add support for GlitchTip to gather more info with errors like Tiki does:
https://dev.tiki.org/Using-GlitchTip-as-part-of-the-Tiki-development-process

This will be especially useful to investivate intermittent issues, like #1282

Probably no need to add by default, but just have some documentation for community members to gather information for bug reports.