makeseeds: update MIN_BLOCKS, add reminder to README

contrib/seeds/makeseeds.py

@@ -26,7 +26,7 @@ MAX_SEEDS_PER_ASN = {
     'ipv6': 10,
 }
 
-MIN_BLOCKS = 840000
+MIN_BLOCKS = 868000
 
 PATTERN_IPV4 = re.compile(r"^((\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})):(\d+)$")
 PATTERN_IPV6 = re.compile(r"^\[([0-9a-z:]+)\]:(\d+)$")