An appendix is a section at the end of your main document that contains supplementary information such as figures, tables, raw data, code, etc.
There are no requirements for having an appendix. Instead, use your judgment to content there that someone might want to see or it might be useful, but it is somewhat extraneous to the report body and/or too detailed to describe inline.
Performance graphs you've drawn that might be interesting but you don't describe/analyze in the results are good candidates.
You might include full copies of the demographics questions and the QoE questions (if those don't appear in the methodology body).
Code (if not straightforward) that you used in your experiments (e.g., a script) perhaps.
You do not have to look for stuff to put in here. Again, if you have content that could be useful but do not describe, consider it for the appendix.
Tip! If you do want to refer the reader to some details in the material in the appendix, that content (at least an excerpt) should probably be in the main report body.