Adding a pointer to kissat repo and version used.

Author: haz

nnf/bin/README.md

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+# Binary files included with python-nnf
+
+## kissat
+
+The `kissat` SAT solver is released under MIT license, and can be found [here](https://github.com/arminbiere/kissat/). The binary included in this directory corresponds to the statically linked version at [this commit](https://github.com/arminbiere/kissat/commit/baef4609163f542dc08f43aef02ce8da0581a2b5). No modifications were made to the `kissat` solver.