Added shell sort program in python

sort/shell_sort.py

@@ -0,0 +1,19 @@
+def shell_sort(arr):
+
+    n = len(arr)
+    # Initialize size of the gap
+    gap = n//2
+
+    while gap > 0:
+        y_index = gap
+        while y_index < len(arr):
+            y = arr[y_index]
+            x_index = y_index - gap
+            while x_index >= 0 and y < arr[x_index]:
+                arr[x_index + gap] = arr[x_index]
+                x_index = x_index - gap
+            arr[x_index + gap] = y
+            y_index = y_index + 1
+        gap = gap//2
+
+    return arr