add explanations of handicap and nominal hashrate multiplier

roles/test-utils/mining-device/README.md

@@ -31,3 +31,30 @@ Usage example:
 ```
 cargo run --release -- --address-pool 127.0.0.1:20000 --id-device device_id::SOLO::bc1qxy2kgdygjrsqtzq2n0yrf2493p83kkfjhx0wlh
 ```
+
+## handicap
+
+CPU mining could damage the system due to excessive heat.
+
+The `--handicap` parameter should be used as a safety mechanism to slow down the hashrate in order to preserve hardware.
+
+## nominal hashrate multiplier
+
+Let's imagine that:
+- the upstream wants to receive shares every ~100s (on average)
+- the CPU miner nominal hashrate is 1k H/s
+
+Maybe we want to do a test where we don't want to wait ~100s before a share is submitted by the CPU miner.
+
+In that case, we need the CPU miner to advertise a smaller hashrate, which will force the upstream to set a lower
+difficulty target.
+
+The `--nominal-hashrate-multiplier` can be used to advertise a custom nominal hashrate.
+
+In the scenario described above, we could launch the CPU miner with `--nominal-hashrate-multiplier 0.01`. 
+
+The CPU miner would advertise 0.01k H/s, which would cause the upstream to set the difficulty target such that the CPU miner would find a share within ~1s.
+
+This feature can also be used to advertise a bigger nominal hashrate by using values above `1.0`.
+
+That can also be useful for testing difficulty adjustment algorithms on Sv2 upstreams.