add README

NonLinearEVP/README.TXT

+A demo of non-linear eigenvalue problems in GetDP.
+
+This model computes one selected eigenfrequency of a grating made of a 
+frequency-dispersive material. Five different formulations are given,
+calling various solvers of the SLEPc library.
+
+Quick start
+-----------
+
+Open `NonLinearEVP.pro' with Gmsh.
+
+Additional info
+---------------
+
+Some documentation is available here: https://arxiv.org/abs/1802.02363
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