Update helpers/README.md

Author: jedisct1

helpers/README.md

@@ -75,7 +75,7 @@ void sodium_add(unsigned char *a, const unsigned char *b, const size_t len);
 
 The `sodium_add()` function accepts two pointers to unsigned numbers encoded in little-endian format, `a` and `b`, both of size `len` bytes.
 
-It computes `a` + `b` in constant time, and overwrites `a` with the result.
+It computes `(a + b) mod 2^(8*len)` in constant time, and overwrites `a` with the result.
 
 This function was introduced in libsodium 1.0.7.