eclipse#457 add a section to make dynamic config source more visible

Signed-off-by: Emily Jiang <emijiang6@googlemail.com>

spec/src/main/asciidoc/configsources.asciidoc

@@ -147,6 +147,17 @@ public class ExampleYamlConfigSourceProvider
 
 Please note that a single `ConfigSource` should be either registered directly or via a `ConfigSourceProvider`, but never both ways.
 
+=== Dynamic ConfigSource
+A dynamic ConfigSource is a `ConfigSource` where its content can be updated, such as updating a property name or value or adding or deleting config properties 
+from it. For the property lookup, the method `config.getValue()` or `config.getOptionalValue()` retrieves the up-to-date value.
+Alternatively, for the injection style, the following lookup should be used to retrieve the up-to-date value.
+[source, text]
+----
+    @Inject
+    @ConfigProperty(name="myprj.some.dynamic.timeout", defaultValue="100")
+    private javax.inject.Provider<Long> timeout; 
+----
+
 === Cleaning up a ConfigSource
 
 If a `ConfigSource` implements the `java.lang.AutoCloseable` interface  then the `close()` method will be called when the underlying `Config` is being released.