contrib: Update location of seeds.txt

Update the steps for updating the hardcoded seed nodes to point to the
new filename on @sipa's server, and add command to decompress it.

Ref: bitcoin#10163

contrib/seeds/README.md

@@ -8,7 +8,7 @@ and remove old versions as necessary.
 
 The seeds compiled into the release are created from sipa's DNS seed data, like this:
 
-    curl -s http://bitcoin.sipa.be/seeds.txt > seeds_main.txt
+    curl -s http://bitcoin.sipa.be/seeds.txt.gz | gzip -dc > seeds_main.txt
     python3 makeseeds.py < seeds_main.txt > nodes_main.txt
     python3 generate-seeds.py . > ../../src/chainparamsseeds.h