Add docs about grabbing supplies

docs/grab_supplies.md

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+# Mosfet Minecraft Bot
+
+## How to Grab Supplies
+
+![a single barrel with two double chests above it](media/grab_supplies.png)
+
+Mosfet will automatically grab supplies he needs to do his job from nearby
+[barrels](https://minecraft.fandom.com/wiki/Barrel#Crafting). Before starting
+the job, he will check each barrel for related supplies and grab some depending
+on the job. He will recheck the barrels for supplies periodically to try and get
+more.
+
+If he can't find any barrels, he will try to continue doing the job anyway. If
+his inventory is too full, he will skip grabbing supplies.
+
+### Advanced Details
+
+The global `g.maximum_supply_slots` in `bot.py` controls the maximum amount of
+used slots allowed while grabbing supplies.
+
+In `jobs.py`, each job sets the `grab_supplies_states.supplies` dictionary to
+specify what items to grab and how many. Here's an example:
+
+```
+self.grab_supplies_states.supplies = {
+	tuple([items.PUMPKIN_ID]): (64, 3),
+}
+```
+
+This tells Mosfet to grab three stacks of pumpkins if he ever has less than 64.