Add readme of azure-spring-boot-metrics-starter. (#760)

* Add readme of azure-spring-boot-metrics-starter.

* Update README.md

* Update README.md

azure-spring-boot-starters/azure-spring-boot-metrics-starter/README.md

@@ -0,0 +1,37 @@
+## Usage
+
+### Add the dependency
+
+`azure-spring-boot-metrics-starter` is published on Maven Central Repository.  
+If you are using Maven, add the following dependency.  
+
+```xml
+<dependency>
+    <groupId>com.microsoft.azure</groupId>
+   <artifactId>azure-spring-boot-metrics-starter</artifactId>
+    <version>2.2.0</version>
+</dependency>
+```
+
+### Add the property setting
+
+Open `application.properties` file and add below properties with your instrumentation key.
+
+```
+management.metrics.export.azuremonitor.instrumentation-key=<your-instrumentation-key-here>
+```
+
+### Trouble Shooting
+1. If the JDK version you use is greater than 1.8, You may meet this problem: 
+```
+NoClassDefFoundError: javax/xml/bind/JAXBException
+```
+
+To solve this issue, you need to add the dependency below into your classpath:
+```
+<dependency>
+   <groupId>javax.xml.bind</groupId>
+   <artifactId>jaxb-api</artifactId>
+   <version>2.3.0</version>
+</dependency>
+```