Add info about caching items

docs/cache_items.md

@@ -4,4 +4,24 @@
 
 ![a single trapped chest with three normal double chests under it](media/cache_items.png)
 
+Mosfet will automatically dump items into the closest trapped chest once his
+inventory gets full enough. Normally this is when the number of used inventory
+slots is >= 27.
 
+If Mosfet can't find a trapped chest nearby, he will place down a normal chest
+if he has one in his inventory. Then he will store the items in it.
+
+Mosfet won't cache items related to his current job. For example, if he's
+farming wood, he will keep axes, beds, chests, and food.
+
+### Advanced Details
+
+The global `g.minimum_cache_slots` in `bot.py` controls the minimum amount of
+used slots needed to trigger item caching.
+
+In `jobs.py`, each job calls `items.set_wanted()` and `items.set_needed()` to
+control which items Mosfet shouldn't cache. No items in the "needed" list will
+be cached and all items in the "wanted" list—except for one stack of each—will
+be cached. The "needed" list is meant for essentials like tools, food, and beds,
+while the "wanted" list is for stuff that's good to have a stack of like seeds
+and saplings.