Add PY4E directory for study of that book

PY4E/ch02/README.md

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+# Chapter 2: Variables, expressions, and statements
+
+This chapter discusses the following topics:
+
+* Values and types
+* Variables
+* Variable names and keywords
+* Statements
+* Operators and operands
+* Expressions
+* Order of operations
+* Modulus operator
+* String operations
+* User input
+* Comments
+* Choosing good variable names
+* Debugging
+