doc: add FTS5 requirement for sqlite backend. Document the requirement for FTS5 if you are using the sqlite backend. Tonu Mikk reported issues with using a SQLite that was missing FTS5 support (RedHat 7 sqlite with python 3.6). Even though FTS5 has been in the sqlite amalgamation since 2015-10-14 (v3.9.0) RedHat chose to not build their package with that. This leads to a traceback when initializing the database.

#!/usr/bin/env python

# Roundup requires Python 2.7+ as mentioned in doc\installation.txt
from __future__ import print_function
import sys

VERSION_NEEDED = (2, 7)

if sys.version_info < VERSION_NEEDED:
    print("Content-Type: text/plain\n")
    print("Roundup requires Python %s.%s or newer." % VERSION_NEEDED)
    sys.exit(0)