Minor Documentation Improvements

[SEI-John]

The documentation has a couple of minor improvements to be made in the section about building a simple application.

1. my_app.m.cpp:20 bsl::cout requires #include <bsl_iostream.h>
2. Under "Setup application build dependencies", option 2, "Create a build workspace with the BDE libraries...", it should mention that the top level CMakeLists.txt will need to be modified to include add_subdirectory(<path_to_bde>)
3. Under "Configure, build and test your application", this command: bbs_build configure build --target my_app needs to be run from where the top level CMakeLists.txt is. It's a little unclear since the previous command cds into the bde directory

Thanks for putting this toolset and documentation together, it's valuable and helpful