add people space docs

I learned about this module from the following MOTD:

> Persona authentication is now available via your .htaccess files. See
https://github.com/gozer-mozilla/mod_authn_persona for more details.

I tried following the instructions in the README, however I could not get LDAP authentication working. After searching on bugzilla, I found [this comment][1] which helped to resolve my issues.

In order to prevent someone else spending their time trying to figure it out, I've documented what needs to be done in order to get this module working on people space.

[1]:https://bugzilla.mozilla.org/show_bug.cgi?id=994114#c10