You can document evidence pretty easily by typing "record NAME" in the developer console. Replace NAME with whatever you want the .dem file to be named. Once you finish documenting any evidence, you will need to disconnect from the server as typing "stop" in console will not suffice in CS:GO.
The newly created demo file will be located in your steam/steamapps/common/Counter-Strike: Global Offensive/csgo/... folder. You can then easily upload the demo as an attachment on the forums.
I hope this helps you gather any future evidence you might need. Until then, the report won't be reviewed.