Add primary command docs

- Relates #755

Author: jvalkeal

spring-shell-docs/src/main/asciidoc/using-shell-customization-singlecommand.adoc

@@ -0,0 +1,23 @@
+[[using-shell-customization-singlecommand]]
+=== Single Command
+ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
+
+If your shell application is made for exactly a single purpose having only one
+command it may be beneficial to configure it for this. Property
+`spring.shell.noninteractive.primary-command` if defined will disable all other
+runners than `NonInteractiveShellRunner` and configures it to use
+defined _Primary Command_.
+
+====
+[source, yaml]
+----
+spring:
+  shell:
+    noninteractive:
+      primary-command: mycommand
+----
+====
+
+For example if you have a command `mycommand` with option `arg`
+it had to be executed with `<shellapp> mycommand --arg hi`, but with above
+setting it can be executed with `<shellapp> --arg hi`.

spring-shell-docs/src/main/asciidoc/using-shell-customization.adoc

@@ -8,3 +8,5 @@ include::using-shell-customization-styling.adoc[]
 include::using-shell-customization-logging.adoc[]
 
 include::using-shell-customization-commandnotfound.adoc[]
+
+include::using-shell-customization-singlecommand.adoc[]