Added custom fixed difficulty example

README.md

@@ -86,6 +86,7 @@ Mainnet test has started! **PPLNS window = 2160 blocks, block time = 10 seconds*
 - Wait until initial p2pool sync is finished, it shouldn't take more than 5-10 minutes. Of course it depends on your connection speed!
 - p2pool has a stratum server listening on port 3333, you can connect xmrig to it now
 - Run `./xmrig -o 127.0.0.1:3333`. Note that you don't need to specify wallet address for xmrig.
+- To set custom fixed difficulty for your miner (for example, 10000), run `./xmrig -u x+10000 -o 127.0.0.1:3333`
 - xmrig should connect and start mining
 - you can connect multiple miners to the same p2pool node. The more the better!
 - From now on, watch your wallet to see if it gets anything