Default behaviour for yaml-cpp install targets missing in README

[rebeccawanner]

I struggled a lot to find out why the targets for yaml-cpp did not get installed, when fetching them like described in the README in the CMakeLists.txt of my own project.

Now I found out, that in this PR (https://github.com/jbeder/yaml-cpp/pull/1007) it was changed to not be always ON by default to install the targets.

It would probably be really helpful to add a small comment to the README under the section 'How to integrate it within your project using CMake' that it could be necessary to add option(YAML_CPP_INSTALL ON) if yaml-cpp is not the top project, because it could save time for other developers as this feels like a pretty standard use case.

If you agree it would be nice to add this.