Add arch to manpage

Author: TheLocehiliosan

yadm.1

@@ -655,6 +655,7 @@ During processing, the following variables are available in the template:
  yadm.class      YADM_CLASS      Locally defined yadm class
  yadm.distro     YADM_DISTRO     lsb_release -si
  yadm.hostname   YADM_HOSTNAME   uname -n (without domain)
+ yadm.arch       YADM_ARCH       uname -m
  yadm.os         YADM_OS         uname -s
  yadm.user       YADM_USER       id -u -n
  yadm.source     YADM_SOURCE     Template filename