Add mu explaination

Author: mehranagh20

handy-algorithms/mu.explain

@@ -0,0 +1,4 @@
+mu[i] = {0 if i contains a prime more than once
+ 1 if i consists of even number of primes
+ -1 otherwise, ie. odd number of primes}
+mu can be used to count number of things with gcd of 1 using inclution exclution