I figured this would be helpful, as I have been asked numerous times how to do it and see others asking the same thing on the forum. So here is how.
First go into your profile. In the top right corner, click on "Edit my Profile."
Once in there, you will have a side bar of numerous options on the left. Click on the on one that says "signature."
From there, you simply type whatever you want your signature to be, and hit "save changes."