added Firefighters tutorial README

Author: BrianWill-Unity

EntitiesSamples/Assets/Tutorials/Firefighters/Common/firefighters.png

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+# Entities Tutorial: Firefighters
+
+A fire spreads across a field, and bots form bucket brigade lines to douse the fire.
+
+![](Common/firefighters.png)
+
+# Step 1: Spawning
+
+This step sets up the core data and spawns the field, the bots, the ponds of water around the field, and the buckets on the field.
+
+# Step 2: Fire simulation
+
+This step adds code to spread the fire. Three solutions are demonstrated:
+
+- A single-threaded solution running on the main thread.
+- A single-threaded solution running in a job.
+- A parallel-threaded solution running in a job.
+
+# Step 3: Bot behaviour
+
+This step adds behiour to the bots, who are organized into teams. Each team forms a line between a pond and the closest fire. A bucket is filled at the pond and then passed up the line.