Add the 2003 M.I.T. licence notice to master (lost in backporting)

[king-aj3]

The master branch currently carries no licence file, and its sources still
carry the 1994 M.I.T. header, which states that distribution is "strictly
prohibited without prior written consent of M.I.T."

The M.I.T.-copyrighted material was re-released in 2003 under permissive
terms, and that text is already in this repository — in
License_and_history/license_for_sources_4.0.txt on the WinMSVS branch,
under "License agreement for the M.I.T. copyrighted material". As you noted
by email, the 2003 notice stopped appearing on master as a result of the
backporting.

This adds a LICENSE.txt at the root of master reproducing that 2003
section verbatim, so the terms travel with the branch as they do on
WinMSVS. The 2003 licence itself requires that "the above copyright notice
and this permission notice must appear in all copies".

Deliberately not included: any statement about the licence covering
FastFieldSolvers' own modifications on this branch. Only you can state that,
and I did not want to presume it.

Also not touched: the 1994 headers in the source files that carry them.
Updating those is a larger and more opinionated change, and it is yours to
make — but they are what a reader hits first, so it may be worth a follow-up.

No obligation to take this; close it freely if you would rather do it your
own way.