Update README with instructions for specifying manifest to buildpack-packager
Update README with instructions for specifying manifest to
buildpack-packager

Signed-off-by: Anthony Emengo <7b3c5a7825e2f856203b8d882230abcb78036abf@pivotal.io>

thesrinivas/php-buildpack,thesrinivas/php-buildpack,thesrinivas/php-buildpack,thesrinivas/php-buildpack,thesrinivas/php-buildpack,thesrinivas/php-buildpack