Create a Python function to search for a substring in a string. The function should take two arguments, the haystack string and the needle string, and should return a boolean indicating whether the substring was found or not.
haystack: "This is a string" 
needle: "is"
def contains_substring(haystack, needle): 
 return needle in haystack

# Usage example
contains_substring("This is a string", "is") # returns True 
contains_substring("This is a string", "xyz") # returns False