Mention environment variables mapping in the guide

 Closes spring-projectsgh-2234

Author: snicoll

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -543,13 +543,18 @@ definitions by simply listing the properties classes directly in the
 
 
 [[boot-features-external-config-relaxed-binding]]
-==== Relaxed binding
+==== Relaxed binding (Properties format)
 Spring Boot uses some relaxed rules for binding `Environment` properties to
 `@ConfigurationProperties` beans, so there doesn't need to be an exact match between
 the `Environment` property name and the bean property name.  Common examples where this
 is useful include underscore separated (e.g. `context_path` binds to `contextPath`), and
 capitalized (e.g. `PORT` binds to `port`) environment properties.
 
+NOTE: Environment variables are usually underscore-separated and upper case so you can
+just use that and Spring Boot will bind it to your bean property names accordingly. For
+instance `MY_PROPERTY` will  match just the same as `myProperty`, `my_property` or
+`my-property`.
+
 Spring will attempt to coerce the external application properties to the right type when
 it binds to the `@ConfigurationProperties` beans. If you need custom type conversion you
 can provide a `ConversionService` bean (with bean id `conversionService`) or custom