Add example in readme.md with compile_commands.json

[phcerdan]

Hi, giving a try to deoplete from YCM.
Can we have an example on how to work with the compile_commands.json. It says no config required... but where should the json be? It is generated by cmake in the build folder, #14 but it should be in the root of the project right? That should be noticed to the user.

Also, for a deoplete noob, do I need something else to say to deoplete? Something like this maybe?
let g:deoplete#sources.cpp = ['buffer', 'clang2']

Thanks!