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.

from roundup.dist.command.build import build_message_files, check_manifest
from setuptools.command.install_lib import install_lib as base

class install_lib(base):

    def run(self):
        check_manifest()
        build_message_files(self)
        base.run(self)