DOC: add troubleshooting entry for lib not found

Thinking about some good warning for gh-6
I wouldn't call it "fix" because in the end it depends on your use case. For example, if you would write a Mac application which uses libraw.dynlib then the relative path may actually be fine, given that the dynlib would be probably end up in the same folder. But for delocate this is a problem because this is a different use case and it actually needs the full path to locate the library, just to remove the full path again. 
I think it would be really good to include the relevant cmake flags as an example, as more and more libraries use it. Then at least you get an idea and can adapt it to other build processes, and more importantly you can google it. So I think a reference to [CMAKE_INSTALL_NAME_DIR](http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_INSTALL_NAME_DIR.html) is enough.