MAINT: add corefined lipid example for refnx

Author: andyfaff

model_library/refnx/CorefinedLipidBilayers/README.md

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+Software
+========
+refnx
+
+Reference
+=========
+“Nelson, A.R.J. & Prescott, S.W. (2019). J. Appl. Cryst. 52, https://doi.org/10.1107/S1600576718017296.”
+
+Description
+===========
+This is an example notebook for how refnx can be used to:
+- corefine multiple contrast datasets
+- study bilayer membranes with physical parameterisation, such as area-per-molecule, head/tail group volumes.
+