Add SPDX information to the README

Yes, the LGPL is a silly licence to use for a README but since it
contains EMBL-copyrighted material I can't do anything else.

TODO: Rewrite the README so it can be CC-BY.

README.md

 # GALUMPH #
 
+<!---
+SPDX-FileCopyrightText: 2016-2017 European Molecular Biology Laboratory (EMBL)
+SPDX-FileCopyrightText: 2018 Christopher Kerr
+
+SPDX-License-Identifier: LGPL-3.0-or-later
+-->
+
 [![pipeline status](https://git.embl.de/grp-svergun/galumph/badges/master/pipeline.svg)](https://git.embl.de/grp-svergun/galumph/commits/master)
 
 Calculate ALM (scattering amplitude decomposed into spherical harmonics) at Hyperspeed using GPU acceleration.
@@ -34,4 +41,4 @@ ff = np.array([periodictable.elements.symbol(aa.element).xray.f0(s) for aa in pd
 alm = kernel.zeros()
 kernel.add_many_atoms(alm, xyz, ff)
 Icalc = kernel.sum_intensity(alm)
-```
\ No newline at end of file
+```