Add note about factory methods

Author: damianmalczewski

docs/02-problem4j-core.md

@@ -72,6 +72,17 @@ try {
 }
 ```
 
+For convenience, `Problem` objects can also be created using static `Problem.of` factory methods.
+
+```java
+static Problem Problem.of(int status);
+static Problem Problem.of(int status, @Nullable String detail);
+static Problem Problem.of(String title, int status);
+static Problem Problem.of(String title, int status, @Nullable String detail);
+static Problem Problem.of(URI type, String title, int status);
+static Problem Problem.of(URI type, String title, int status, @Nullable String detail);
+```
+
 ## Throw exception annotated with `@ProblemMapping`
 
 If an exception is annotated with `@ProblemMapping`, extracting the underlying `Problem` from it requires a bit of

docs/04-problem4j-spring/02-exception-handling.md

@@ -222,6 +222,17 @@ Will result in following response body:
 }
 ```
 
+For convenience, `Problem` objects can also be created using static `Problem.of` factory methods.
+
+```java
+static Problem Problem.of(int status);
+static Problem Problem.of(int status, @Nullable String detail);
+static Problem Problem.of(String title, int status);
+static Problem Problem.of(String title, int status, @Nullable String detail);
+static Problem Problem.of(URI type, String title, int status);
+static Problem Problem.of(URI type, String title, int status, @Nullable String detail);
+```
+
 ## Spring's build-in `@ResponseStatus` annotation
 
 If your exception is annotated with Spring's built-in `@ResponseStatus`, the library will use the specified HTTP status