Adds README

Author: samagra14

README.md

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+## Combinatorial_lib for java
+The aim of this library is to provide an interface
+ similar to `itertools` in python.
+
+### Example Usages
+#### Combinations
+To iterate over all combinations of `k` objects from a `List<T> list` just use
+`CombinationGenerator.generateCombinations(list,k)`
+````java
+for (List<T> possibleCombinations :
+                CombinationGenerator.generateCombinations(list,k)) {
+            //perform operations on your combinations
+        }
+````
+#### Permutations
+To iterate over all permutations of `k` objects from a `List<T> list` just use
+`CombinationGenerator.generateCombinations(list,k)`
+````java
+for (List<T> possiblePermutations :
+                PermutationGenerator.generatePermutations(list,k)) {
+            //perform operations on your permutations
+        }
+````