Mention that completion doesn't work for all versions of shells

(.venv) [vagrant@p2 check_patroni]$ eval "$(_CHECK_PATRONI_COMPLETE=bash_source check_patroni)"        
Traceback (most recent call last):
  File "/home/vagrant/check_patroni/.venv/bin/check_patroni", line 11, in <module>
    load_entry_point('check-patroni', 'console_scripts', 'check_patroni')()
  File "/home/vagrant/check_patroni/.venv/lib64/python3.6/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/home/vagrant/check_patroni/.venv/lib64/python3.6/site-packages/click/core.py", line 1048, in main
    self._main_shell_completion(extra, prog_name, complete_var)
  File "/home/vagrant/check_patroni/.venv/lib64/python3.6/site-packages/click/core.py", line 1123, in _main_shell_completion
    rv = shell_complete(self, ctx_args, prog_name, complete_var, instruction)
  File "/home/vagrant/check_patroni/.venv/lib64/python3.6/site-packages/click/shell_completion.py", line 45, in shell_complete
    echo(comp.source())
  File "/home/vagrant/check_patroni/.venv/lib64/python3.6/site-packages/click/shell_completion.py", line 326, in source
    self._check_version()
  File "/home/vagrant/check_patroni/.venv/lib64/python3.6/site-packages/click/shell_completion.py", line 316, in _check_version
    "Shell completion is not supported for Bash"
RuntimeError: Shell completion is not supported for Bash versions older than 4.4.
(.venv) [vagrant@p2 check_patroni]$ bash --version
GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
dalibo / check_patroni

Mention that completion doesn't work for all versions of shells #53
