Added note regarding pip requirements
Added note regarding pip requirements

Since the project switched to PEP517 installation
only, it is required to have pip 19.0+ installed.
Earlier versions of pip don't support PEP517 style
installation.

Added note to README to inform about this

Relates to #643

Signed-off-by: Konrad Weihmann <237a31e24baec6fd7412e435f5dfb93997617781@outlook.com>

willthames/ansible-lint