Add use of safe_load() function in README (#285)
Add use of safe_load() function in README (#285)

It helps people to use `safe_load` if they discover the library.
It's more secure if `safe_load()` is used by default, and `load()` is used if it's necessary (and the developer knows what is does).
yaml/pyyaml,anishathalye/pyyaml,yaml/pyyaml,yaml/pyyaml,anishathalye/pyyaml