comment why MASTER_SITES isn't assigned in the usual way

The pkgsrc guide says to do:

  MASTER_SITES=${MASTER_SITE_SOURCEFORGE:=project_name/}

but this doesn't work if project_name/ contains a % character (e.g., a
space in the URL that was encoded to %20) because % is a special
control character.  So this Makefile instead does:

  MASTER_SITES=${MASTER_SITE_SOURCEFORGE@.SITE.@${.SITE.}project_name/}

to accomplish the same thing without the % limitation.  Add a comment
explaining this.