Update ThingsToKnow.rst

[rootless4real]

uWSGI is picky about content being returned via CGI scripts. Across days and days of searching I finally found this: https://stackoverflow.com/questions/22543334/send-perl-cgi-request-and-get-xml-as-output which speaks to it tangentially. A significant enough issue that doesn't afflict running with mod_perl, making it much, much harder to troubleshoot and worth of a mention.