build: pass --no-xmllint to a2x

On some platforms a2x fails with:

a2x: ERROR: xmllint --nonet --noout --valid /lpcmdline/lpass.1.xml returned non-zero exit status 4

Without --nonet, it downloads the dtd and works, but there's no
real need to validate the asciidoc output, so just disable it.

Signed-off-by: Bob Copeland <copeland@lastpass.com>
doc: make a note about the documentation target

Point out that we do have a manpage, for those that decide to
read the README.

Signed-off-by: Bob Copeland <copeland@lastpass.com>
doc: add language requiring DCO

To ensure provenance and license acceptance, ask people to sign
their contributions.

Signed-off-by: Bob Copeland <copeland@lastpass.com>