Write a simple README

Author: Filippo Costa

README.md

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+# λ²
+λ² is a λ-calculus implementation written only in Python lambdas.
+
+- No recursion.
+- No Python built-ins (integers, strings, conditionals... nothing!).
+- No external dependencies. (Duh.)
+
+### What is this?
+
+From [the Wikipedia entry](https://en.wikipedia.org/wiki/Lambda_calculus) on λ-calculus:
+
+> Lambda calculus (also written as λ-calculus) is a formal system in mathematical logic for expressing computation based on function abstraction and application using variable binding and substitution. It is a universal model of computation that can be used to simulate any single-taped Turing machine and was first introduced by mathematician Alonzo Church in the 1930s as part of his research of the foundations of mathematics.
+
+Binary λ-calculus is just a convenient encoding for already valid λ-calculus programs. `00` is abstraction and `01` is application. Instead of variables, binary λ-calculus uses [De Bruijn indexes](https://en.wikipedia.org/wiki/De_Bruijn_index).
+
+### Why?
+Why not?