updated credits.md

Author: GioF71

credits.md

@@ -1,4 +1,9 @@
 # Credits
 
-## Google Java Style Guide
+## Style 
+### Google Java Style Guide
 The html file available here has been copied from [this page](https://google.github.io/styleguide/javaguide.html).
+
+## Solid
+### Single-responsability
+[This](https://en.wikipedia.org/wiki/Single-responsibility_principle) article from [Wikipedia](https://wikipedia.org).