To everyone grousing about BoingBoing publicizing Chelsea's mailing address, it's fairly much tradition for supporters of political prisoners or the wrongfully imprisoned to give out mailing addresses. People in prison have limited contact to the outside world and it's nice to hear from supporters.
If you have nice things to say to Chelsea and are going to use her correct pronouns inside the letter, please do. As oldsma mentioned, she did say she'd like correspondence.
It's often that way for civilian prisons too. People who write CeCe McDonald letters have to use the name the prison has her down for.