Add documentation about plog and conan

[SergiusTheBest]

plog is now available on conan-center with an official recipe as per https://github.com/conan-io/conan-center-index/tree/master/recipes/plog/all

Originally posted by @macdew in https://github.com/SergiusTheBest/plog/issues/75#issuecomment-796858009