added pointer to LucasChainGenerator/README

NEWS

 Changes between GMP-ECM 7.0.5 and 7.0.6:
 * added a generator for optimal Lucas chains for param=0
-  (see README)
+  (see README and LucasChainGenerator/README)
 * the option -I has been removed
   (https://gitlab.inria.fr/zimmerma/ecm/-/issues/21867)
 * fixed a bug when using -resume with param=0 and B1 > B1 in resume file