Document and explain messages reports

Author: badeball

docs/messages-report.md

@@ -0,0 +1,13 @@
+# Messages reports
+
+"Messages" refers to [Cucumber Messages](https://github.com/cucumber/common/tree/main/messages) and is a low-level report / protocol for representing results and other information from Cucumber. JSON reports and HTML reports are derived from this report. Hence, messages report will implicitly be enabled if either of the mentioned reports are.
+
+Messages reports can be enabled using the `messages.enabled` property. The preprocessor uses [cosmiconfig](https://github.com/davidtheclark/cosmiconfig), which means you can place configuration options in EG. `.cypress-cucumber-preprocessorrc.json` or `package.json`. An example configuration is shown below.
+
+```json
+{
+  "messages": {
+    "enabled": true
+  }
+}
+```

docs/readme.md

@@ -6,8 +6,10 @@
 * [State management](state-management.md)
 * [Step definitions](step-definitions.md)
 * [Tags](tags.md)
-* [JSON report](json-report.md)
-* [HTML report](html-report.md)
+* Reports
+  * [Messages report](messages-report.md)
+  * [JSON report](json-report.md)
+  * [HTML report](html-report.md)
 * [Localisation](localisation.md)
 * [Configuration](configuration.md)
 * [Test configuration](test-configuration.md)