[github] Add sponsorship link

[github] Add sponsorship link

iPXE is released under the GNU GPL and is 100% open source software.
There are no "premium editions", no in-app advertisements, and no
hidden costs.  The fully public version published to GitHub is and
always will be the definitive and only version of iPXE.

Many large features in iPXE have been commercially funded within this
open source model, with features being published upstream as soon as
they are complete and made available for the whole world to use, not
restricted for use only by the customer funding that particular piece
of development work.

There has not to date been any funding model for smaller pieces of
work, such as occasional code review or guaranteed attention to bug
reports.  The overhead of establishing a commercial relationship is
usually too high to be worthwhile for very small units of work.

The GitHub sponsorship mechanism provides a framework for efficiently
handling small commercial requests (or individual tokens of thanks).
Add a FUNDING.yml file to provide a convenient way for anyone who
wants to support the ongoing open source development of iPXE to do so.

Signed-off-by: Michael Brown <mcb30@ipxe.org>