An Imgur user really wanted to know what Sasna wrote in her letter. So, he did an amazing job to decipher it.
From this angle you can makeout “to fufill”, “for Winterfell” and “rewarded.”
This scene is where most of the message comes in.
*CSI voice* Enhance!
“[…] to protect me. Now you have a chance to fulfill your promise. […] Knights of the Vale are under your command. Ride north for Winterfell. Lend us your aid and I shall see to it that you are [well/properly] rewarded.”