Windows Defender is and idiot, sorry to say. License.rtf is a rich text file containing our license text. It is not executable. The only way this could harm your computer would be if the rich text viewer you are using to display the file has a bug.
Programs like Windows Defender work based on something called "heuristics" (among other things). That is, they know a few byte patterns they look for, irrespective of where they find them in the files of a program. What you see here is called a "false positive", meaning, it detects something as dangerous, while it is not.
Spine does not do anything malicious, despite what Windows Defender's heuristics tell you. We guarantee that in our license agreement, so if we actually did, you could sue us into the ground.
TL;DR: don't trust spurious outputs from your "anti-virus" software.