Add some documentation on external sources

README.md

@@ -39,7 +39,32 @@ submission and querying. The query API is a simple IP lookup,
 returning a score. This conceivably could be turned into a DNS-based
 API as well.
 
-## Third-party sources
+## External sources
 
-It would be nice to allow the scoring script to consult other IP-based
-third-party sources, such as DNSBLs, or GeoIP lookups, etc.
+The scoring script can consult other IP-based third-party sources, such
+as DNSBLs, or GeoIP lookups, etc.
+
+These are configured via YAML snippets in a directory, each file
+corresponding to a separate external source.
+
+Each source configuration should specify the following parameters:
+
+* *name*, the name of the source
+* *type*, one of the supported source types (either *dnsbl* or *geoip*
+  at the moment)
+* *params*, a dictionary of further type-specific configuration
+  parameters.
+
+The parameters for *dnsbl* source type are:
+
+* *domain*, the DNSBL domain to query
+* *match*, a regexp pattern for the DNS query result. The default is
+  ".*", i.e. any result will be regarded as a positive match.
+
+The parameters for *geoip* source type are:
+
+* *paths*, a list of paths to MaxMind GeoIP databases. The default
+  list contains a single path, /var/lib/GeoIP/GeoLite2-Country.mmdb
+
+External sources can be used in the scoring script by calling the
+*ext()* function, with the source name and IP address as parameters.

ext/ext.go

@@ -29,7 +29,7 @@ func New(sourceType string, params map[string]interface{}) (ExternalSource, erro
 		}
 		match, ok := params["match"].(string)
 		if !ok {
-			return nil, errors.New("missing parameter 'match'")
+			match = ".*"
 		}
 		return dnsbl.New(domain, match)