add notice to README about release lines; include link to README for 1.5.x [skip ci]
add notice to README about release lines; include link to README for 1.5.x [skip ci]

mojavelinux/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,asciidoctor/asciidoctor-pdf