Added Footnotes.addAfterFailure(..) to user guide.

See #310

Author: jlink

documentation/src/docs/include/writing-properties.md

@@ -177,7 +177,7 @@ public class FootnotesExamples {
 ```
 
 Unlike standard reporting, the footnotes feature must be explicitly enabled through
-the annotation `EnableFootnotes`, which can be added to container classes or individual property methods.
+the annotation `@EnableFootnotes`, which can be added to container classes or individual property methods.
 Now you can add a parameter of type `net.jqwik.api.footnotes.Footnotes` to a property method
 or a lifecycle method annotated with either `@BeforeTry` or `@AfterTry`.
 The footnote string will then be part of the sample reporting:
@@ -200,6 +200,10 @@ Original Sample
   #1 399322
 ```
 
+For footnotes that require significant computation you can also use 
+`Footnotes.addAfterFailure(Supplier<String> footnoteSupplier)`.
+Those suppliers will only be evaluated if the property fails, and then as early as possible.
+Mind that this evaluation can still happen quite often during shrinking.
 
 ### Optional `@Property` Attributes
 

documentation/src/test/java/net/jqwik/docs/footnotes/FootnotesExamples.java

@@ -15,4 +15,11 @@ void differenceShouldBeBelow42(@ForAll int number1, @ForAll int number2, Footnot
 		Assertions.assertThat(difference).isLessThan(42);
 	}
 
+	@Property
+	void lazyFootnotes(@ForAll int anInt, @ForAll boolean shouldFail, Footnotes footnotes) {
+		footnotes.addAfterFailure(() -> "anInt=" + anInt);
+		footnotes.addAfterFailure(() -> "shouldFail=" + shouldFail);
+		Assertions.assertThat(shouldFail).isFalse();
+	}
+
 }