Added a sender that works in a reactive environment; fixes gh-2106

docs/src/main/asciidoc/project-features.adoc

@@ -286,6 +286,8 @@ The following example shows setting the sender type for `web`:
 spring.zipkin.sender.type: web
 ----
 
+If you're running a non-reactive application we will use a `RestTemplate` based span sender. Otherwise a `WebClient` based span sender will be chosen.
+
 To customize the `RestTemplate` that sends spans to Zipkin via HTTP, you can register the `ZipkinRestTemplateCustomizer` bean.
 
 [source,java,indent=0]
@@ -321,6 +323,17 @@ By default, api path will be set to `api/v2/spans` or `api/v1/spans` depending o
 spring.zipkin.api-path: v2/path2
 ----
 
+In case of a reactive application, we're creating a simple `WebClient.Builder` instance. If you want to provide your own or reuse an existing one you need to create an instance of a `ZipkinWebClientBuilderProvider` bean.
+
+[source,java,indent=0]
+----
+	@Bean
+	ZipkinWebClientBuilderProvider myZipkinWebClientBuilderProvider() {
+	    // create your own instance or inject one from the Spring Context
+		return () -> WebClient.builder();
+	}
+----
+
 [[features-zipkin-custom-service-name]]
 === Custom service name