seeds: Add testnet instructions

contrib/seeds/.gitignore

@@ -1,2 +1,3 @@
 seeds_main.txt
+seeds_test.txt
 asmap-filled.dat

contrib/seeds/README.md

@@ -14,8 +14,12 @@ data. Run the following commands from the `/contrib/seeds` directory:
 ```
 curl https://bitcoin.sipa.be/seeds.txt.gz | gzip -dc > seeds_main.txt
 curl https://mainnet.achownodes.xyz/seeds.txt.gz | gzip -dc >> seeds_main.txt
+curl https://testnet.achownodes.xyz/seeds.txt.gz | gzip -dc > seeds_test.txt
 curl https://bitcoin.sipa.be/asmap-filled.dat > asmap-filled.dat
 python3 makeseeds.py -a asmap-filled.dat -s seeds_main.txt > nodes_main.txt
 cat nodes_main_manual.txt >> nodes_main.txt
+python3 makeseeds.py -a asmap-filled.dat -s seeds_test.txt > nodes_test.txt
+# TODO: Uncomment when a seeder publishes seeds.txt.gz for testnet4
+# python3 makeseeds.py -a asmap-filled.dat -s seeds_testnet4.txt -m 30000 > nodes_testnet4.txt
 python3 generate-seeds.py . > ../../src/chainparamsseeds.h
 ```