Update document for Sentinel

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

docs/src/main/asciidoc/spring-cloud-circuitbreaker-sentinel.adoc

@@ -0,0 +1,44 @@
+=== Configuring Sentinel Circuit Breakers
+
+==== Default Configuration
+
+To provide a default configuration for all of your circuit breakers create a `Customizer` bean that is passed a
+`SentinelCircuitBreakerFactory`.
+The `configureDefault` method can be used to provide a default configuration.
+
+====
+[source,java]
+----
+@Bean
+public Customizer<SentinelCircuitBreakerFactory> defaultCustomizer() {
+	return factory -> factory.configureDefault(id -> new SentinelConfigBuilder(id)
+			.build());
+}
+----
+====
+
+You can choose to provide default circuit breaking rules via `SentinelConfigBuilder#rules(rules)`.
+You can also choose to load circuit breaking rules later elsewhere using
+`DegradeRuleManager.loadRules(rules)` API of Sentinel.
+
+
+==== Specific Circuit Breaker Configuration
+
+Similarly to providing a default configuration, you can create a `Customizer` bean this is passed a
+`SentinelCircuitBreakerFactory`.
+
+====
+[source,java]
+----
+@Bean
+public Customizer<SentinelCircuitBreakerFactory> slowCustomizer() {
+	String slowId = "slow";
+	List<DegradeRule> rules = Collections.singletonList(
+		new DegradeRule(slowId).setGrade(RuleConstant.DEGRADE_GRADE_RT)
+			.setCount(100)
+			.setTimeWindow(10)
+		);
+	return factory -> factory.configure(builder -> builder.rules(rules), slowId);
+}
+----
+====

docs/src/main/asciidoc/spring-cloud-circuitbreaker.adoc

@@ -20,6 +20,7 @@ breaker implementation that best fits your needs for your app.
 
 * https://github.com/Netflix/Hystrix[Netfix Hystrix]
 * https://github.com/resilience4j/resilience4j[Resilience4J]
+* https://github.com/alibaba/Sentinel[Sentinel]
 
 == Core Concepts
 
@@ -96,6 +97,9 @@ include::spring-cloud-circuitbreaker-hystrix.adoc[]
 
 include::spring-cloud-circuitbreaker-resilience4j.adoc[]
 
+
+include::spring-cloud-circuitbreaker-sentinel.adoc[]
+
 == Building
 
 include::https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/docs/src/main/asciidoc/building-jdk8.adoc[]