Add to Testing docs to make more clear

When going through the docs, it is not clear that binary files cannot be tested. Additionally, it is hard to find the proper structure of a Rust crate and it took me several hours of looking through the docs to find the crates and modules section. I think we can link to it from here and it will be beneficial to those who are coming to the language.

Author: kickinbahk

src/doc/trpl/testing.md

@@ -504,3 +504,5 @@ you add more examples.
 
 We haven’t covered all of the details with writing documentation tests. For more,
 please see the [Documentation chapter](documentation.html)
+
+One final note: Tests *cannot* be run on the binary (main) file. To see more on file arrangement see the [Crates and Modules](https://doc.rust-lang.org/stable/book/crates-and-modules.html) Section