add links and citation

Author: dbc2201

.idea/workspace.xml

@@ -14,8 +14,19 @@ Write a program that counts duplicate characters from a given `String`.
 "aabcc"
 ```
 
-### Sample Output 2
+### Sample Output 1
 
 ```
 {a=2, b=1, c=2}
-```
+```
+
+---
+
+* [Source Code](src/main/java/io/github/dbc/DuplicateCharactersCounter.java)
+* [Main Class](src/main/java/io/github/dbc/Main.java)
+* [Test Class](src/test/java/io/github/dbc/DuplicateCharactersCounterTest.java)
+
+---
+> Question taken from the book
+> ["Java Coding Problems"](https://www.packtpub.com/product/java-coding-problems/9781789801415) by Anghel Leonard.  
+> Formatted and uploaded by [dbc2201](https://github.com/dbc2201)