documentation_vs_reality--PDI--libyaml_requirement

LibYAML is a transitive requirement of PDI via paraconf. It is not mentioned as a prerequisite for building PDI (I imagine) since paraconf ships with a vendored version of LibYAML. However, paraconf defaults to USE_SYSTEM_YAML = ON, so when I execute the step by step build instructions of PDI on a system where LibYAML is not installed, cmake tells me that it cannot find LibYAML. Adding -D USE_SYSTEM_YAML=OFF to the invocation of cmake gets around this. Not a big issue, but I think this could lead to some irritation and is easily fixed by either documenting it or changing the default behavior in paraconf to either default to the vendored LibYAML or (maybe even better) fall back to the vendored LibYAML if it does not find a system LibYAML.