[deepl-java-1.0.1] Describe Translator thread-safety in documentation

[pfoermer]

Hi,

in the README.md of https://github.com/DeepLcom/deepl-java and the class header javadoc of Translator I could not find directly an information whether a Translator instance is thread-safe or not and can be shared between different threads. Would it be possible to add a note for that?

Thanks and best regards,
Philipp Förmer