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.

try:
    # Python 3+
    from http import client, server
    server.DEFAULT_ERROR_MESSAGE
except (ImportError, AttributeError):
    # Python 2.5-2.7
    import httplib as client                     # noqa: F401
    import BaseHTTPServer as server              # noqa: F401