contrib/seeds: Update PATTERN_AGENT

gmaxwell: Adds the newest versions, drops everything multiple softforks ago.

contrib/seeds/README.md

@@ -3,6 +3,9 @@
 Utility to generate the seeds.txt list that is compiled into the client
 (see [src/chainparamsseeds.h](/src/chainparamsseeds.h) and other utilities in [contrib/seeds](/contrib/seeds)).
 
+Be sure to update `PATTERN_AGENT` in `makeseeds.py` to include the current version,
+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