docs: document how to extend

docs/extend-assert-baseline.md

@@ -1,2 +1,21 @@
 # Extend with another baseline format
-[TODO]
+
+You can extend [AbstractBaselineAssertion](../src/main/java/io/github/goatfryed/assert_baseline/core/AbstractBaselineAssertion.java)
+to implement your own format.
+
+Compare [JsonBaselineAssertion](../src/main/java/io/github/goatfryed/assert_baseline/json/JsonBaselineAssertion.java)
+
+(!) This internal API is considered unstable.
+
+## Extending from AbstractBaselineAssertion
+
+### 1. implement write
+Implement `AbstractBaselineAssertion::saveActual(BaselineContext)`.
+Serialize your subject and write it to `BaselineContext::getActualOutputStream`
+
+### 2. implement verification
+Implement `verifyIsEqualToBaseline(BaselineContext context)`.
+Usually, you'll want to read from `BaselineContext::getBaselineAsString(), deserialize the baseline,
+and then compare the java beans.
+
+### implement further conventional methods

src/main/java/io/github/goatfryed/assert_baseline/core/AbstractBaselineAssertion.java

@@ -39,8 +39,14 @@ public final SELF usingStorage(Configurer<StorageFactory> configurer) {
         return myself;
     }
 
+    /**
+     * Expects you to write your subject in serialized form to {@link BaselineContext#getActualOutputStream()}
+     */
     abstract protected void saveActual(BaselineContext context);
 
+    /**
+     * Usually, you'll want to read {@link BaselineContext#getBaselineAsString()}, deserialize the baseline and compare the models.
+     */
     abstract protected void verifyIsEqualToBaseline(BaselineContext context);
 
     private BaselineContextFactory getContextFactory() {