Add AnturK's 'GC Issues' guide

code/controllers/subsystem/garbage.dm

@@ -1,3 +1,26 @@
+/*!
+## Debugging GC issues
+
+In order to debug `qdel()` failures, there are several tools available.
+To enable these tools, define `TESTING` in [_compile_options.dm](https://github.com/tgstation/-tg-station/blob/master/code/_compile_options.dm).
+
+First is a verb called "Find References", which lists **every** refererence to an object in the world. This allows you to track down any indirect or obfuscated references that you might have missed.
+
+Complementing this is another verb, "qdel() then Find References".
+This does exactly what you'd expect; it calls `qdel()` on the object and then it finds all references remaining.
+This is great, because it means that `Destroy()` will have been called before it starts to find references,
+so the only references you'll find will be the ones preventing the object from `qdel()`ing gracefully.
+
+If you have a datum or something you are not destroying directly (say via the singulo),
+the next tool is `QDEL_HINT_FINDREFERENCE`. You can return this in `Destroy()` (where you would normally `return ..()`),
+to print a list of references once it enters the GC queue.
+
+Finally is a verb, "Show qdel() Log", which shows the deletion log that the garbage subsystem keeps. This is helpful if you are having race conditions or need to review the order of deletions.
+
+Note that for any of these tools to work `TESTING` must be defined.
+By using these methods of finding references, you can make your life far, far easier when dealing with `qdel()` failures.
+*/
+
 SUBSYSTEM_DEF(garbage)
 	name = "Garbage"
 	priority = FIRE_PRIORITY_GARBAGE