[archiso] Document mirror= bootparam

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>
[archiso] Document isohybrid-gpt boot method

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>
[archiso] README.build update

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>