Use GetDesireableServiceFlags in static seeds, document this.

44407100f broke inserting entries into addrman from static seeds
(as well as dnsseeds which did not support service bits). Static
seeds were already being filtered by UA for 0.13.1+ (ie
NODE_WITNESS), so simply changing the default service bits to
include NODE_WITNESS (and updating docs appropriately) is
sufficient.

For DNS Seeds, we will later fix by falling back to oneshot if a
seed does not support filtering.

contrib/seeds/README.md

@@ -4,7 +4,9 @@ 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.
+and remove old versions as necessary (at a minimum when GetDesireableServiceFlags
+changes its default return value, as those are the services which seeds are added
+to addrman with).
 
 The seeds compiled into the release are created from sipa's DNS seed data, like this: