Document possible time zone values (#147)

This might be a normal expectation, but it was new for me.

I added a URL to fast-track the next person in the same position.

TEMPLATE.env

@@ -33,6 +33,8 @@ ACCOUNT_SIGNUPS_ENABLED=False
 ACCOUNT_EMAIL_VERIFICATION=none
 
 # The timezone of the admin panel. Affects how dates are displayed.
+# This must match a value from the IANA's tz database.
+# Wikipedia has a list of valid strings: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
 TIME_ZONE=America/New_York
 
 # Set to "False" if you will not be serving content over HTTPS