add README file

Author: fblanqui

README.md

@@ -0,0 +1,11 @@
+Lambdapi library on natural numbers and polymorphic lists
+=========================================================
+
+- Set: type of set codes
+- Prop: definition of propositional logic
+- Eq: definition of Leibniz equality
+- FOL: definition of first-order logic
+- Bool: definition of booleans
+- Nat: definition and properties of natural numbers
+- List: definition and properties of polymorphic lists
+- NatBool: some boolean functions on natural numbers