Added a book on Clean ABAP (#3442)

Added the book "SAP Code Style Guides - Clean ABAP", which is an adoption of Robert C. Martin's "Clean Code" for the language ABAP. The license is CC BY 3.0.

free-programming-books.md

 ### Index
 
+* [ABAP](#abap)
 * [Ada](#ada)
 * [Agda](#agda)
 * [Alef](#alef)
@@ -785,6 +786,11 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Mature Optimization](http://carlos.bueno.org/optimization/mature-optimization.pdf) - Carlos Bueno (PDF)
 
 
+### ABAP
+
+* [SAP Code Style Guides - Clean ABAP](https://github.com/SAP/styleguides/blob/master/clean-abap/CleanABAP.md)
+
+
 ### Ada
 
 * [A Guide to Ada for C and C++ Programmers](http://www.cs.uni.edu/~mccormic/4740/guide-c2ada.pdf) (PDF)