Update README.md

In the recent specification PR (w3c/ServiceWorker#1686), the urlPattern condition accepts URLPatternInput instead of USVString.  This CL adjusted the explanation.

Also, this CL made RouterCondition to have the urlPattern field
for ease of adding other fields as "and" condition.

README.md

@@ -84,11 +84,9 @@ dictionary RouterRule {
 
 // Defines when to respond from the source.
 dictionary RouterCondition {
-};
-
-dictionary RouterUrlPatternCondition : RouterCondition {
-  // a relative path pattern string usable for URLPattern.
-  USVString urlPattern;
+  // https://wicg.github.io/urlpattern/#typedefdef-urlpatterninput
+  // For a USVString input, a ServiceWorker script's URL is used as a base URL.
+  URLPatternInput urlPattern;
 };
 
 enum RouterSourceEnum { "network" };