Add image banner documentation

Closes gh-4647

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -31,6 +31,11 @@ content into your application; rather pick only the properties that you need.
 	# BANNER
 	banner.charset=UTF-8 # Banner file encoding.
 	banner.location=classpath:banner.txt # Banner file location.
+	banner.image.location=classpath:banner.gif # (Banner image file location, jpg/png can also be used).
+	banner.image.width= # Width of the banner image in chars (default 76)
+	banner.image.hight= # Height of the banner image in chars (default based on image height)
+	banner.image.margin= # Left hand image margin in chars (default 2)
+	banner.image.invert= # If images should be inverted for dark terminal themes (default false)
 
 	# LOGGING
 	logging.config= # Location of the logging configuration file. For instance `classpath:logback.xml` for Logback

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

@@ -212,8 +212,8 @@ might have.
 
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
-	spring.main.web_environment=false
-	spring.main.banner_mode=off
+	spring.main.web-environment=false
+	spring.main.banner-mode=off
 ----
 
 and then the Spring Boot banner will not be printed on startup, and the application will
@@ -239,7 +239,7 @@ used with the following configuration:
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
 	spring.main.sources=com.acme.Config,com.acme.ExtraConfig
-	spring.main.banner_mode=console
+	spring.main.banner-mode=console
 ----
 
 The actual application will _now_ show the banner (as overridden by configuration) and use

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

@@ -52,8 +52,11 @@ such as the user that launched the application.
 The banner that is printed on start up can be changed by adding a `banner.txt` file
 to your classpath, or by setting `banner.location` to the location of such a file.
 If the file has an unusual encoding you can set `banner.charset` (default is `UTF-8`).
+In addition a text file, you can also add a `banner.gif`, `banner.jpg` or `banner.png`
+image file to your classpath, or set a `banner.image.location` property. Images will be
+converted into an ASCII art representation and printed above any text banner.
 
-You can use the following variables inside your `banner.txt` file:
+Inside your `banner.txt` file you can use any of the following placeholders:
 
 .Banner variables
 |===