Added docs about logback-spring and props

we had a couple of questions related to missing attributes in the logback file or the logs as such. It's related to the fact that once you provide a custom logback file you have to place the propery of spring.application.name in the bootstrap file

fixes #376

docs/src/main/asciidoc/intro.adoc

@@ -158,6 +158,9 @@ Below you can find an example of a Logback configuration (file named `https://gi
 include::https://raw.githubusercontent.com/spring-cloud-samples/sleuth-documentation-apps/master/service1/src/main/resources/logback-spring.xml[]
 -----
 
+NOTE: If you're using a custom `logback-spring.xml` then you have to pass the `spring.application.name` in
+`bootstrap` instead of `application` property file. Otherwise your custom logback file won't read the property properly.
+
 === Adding to the project
 
 ==== Only Sleuth (log correlation)