Document XInclude support as a FAQ

[hartwork]

Asked at:

• XInclude libexpat#148
• https://www.c-plusplus.net/forum/234446-full
• https://mail.python.org/pipermail/expat-discuss/2002-July/000544.html

Current best answer:

• Build on top of Expat yourself using a start/element handler
• See https://github.com/libexpat/libexpat/blob/master/expat/doc/reference.html#L751 also
• (https://github.com/01org/opa-fm may have code to do just that at some point)