Add the transform examples for the std::reduce section.

CPP17.md

@@ -560,6 +560,15 @@ std::reduce(std::cbegin(a), std::cend(a)); // == 6
 // Using a custom binary op:
 std::reduce(std::cbegin(a), std::cend(a), 1, std::multiplies<>{}); // == 6
 ```
+Additionally you can specify transformations for reducers:
+```c++
+std::transform_reduce(std::cbegin(a), std::cend(a), 0, std::plus<>{}, times_ten); // == 60
+
+const std::array<int, 3> b{ 1, 2, 3 };
+const auto product_times_ten = [](const auto a, const auto b) { return a * b * 10; };
+
+std::transform_reduce(std::cbegin(a), std::cend(a), std::cbegin(b), 0, std::plus<>{}, product_times_ten); // == 140
+```
 
 ### Prefix sum algorithms
 Support for prefix sums (both inclusive and exclusive scans) along with transformations.