Merge pull request #415 from stan-dev/external-fns-includes

Add includes to external function example

Author: rok-cesnovar

src/cmdstan-guide/external_code.Rmd

@@ -42,6 +42,9 @@ this program to C++, but the generated C++ code will not compile
 unless you write a file
 such as `examples/bernoulli/make_odds.hpp` with the following lines
 ```
+#include <boost/math/tools/promotion.hpp>
+#include <ostream>
+
 namespace bernoulli_model_namespace {
           template <typename T0__>  inline  typename
           boost::math::tools::promote_args<T0__>::type  make_odds(const T0__&