Document how to use the builder options

docs/content/en/docs/Advanced Features/builder_options.md

@@ -0,0 +1,35 @@
+---
+title: "Builder options"
+description: >-
+  Advanced options applied when writing the generated code
+---
+
+The `moor_generator` package has some options that control how the 
+code is generated. Note that, in most cases, the default settings
+should be sufficient.
+
+To use the options, create a `build.yaml` file in the root of your project (e.g. next
+to your `pubspec.yaml`):
+```yaml
+# build.yaml. This file is quite powerful, see https://pub.dev/packages/build_config
+
+targets:
+  $default:
+    builders:
+      moor_generator:
+        options:
+          generate_private_watch_methods: true
+```
+
+At the moment, moor supports these options:
+
+* `generate_private_watch_methods`: boolean. There was a bug in the generator where
+  [compiled queries]({{<relref "../Using SQL/custom_queries.md">}}) that start with
+  an underscore did generate a watch method that didn't start with an underscore
+  (see [#107](https://github.com/simolus3/moor/issues/107)). Fixing this would be
+  a breaking change, so the fix is opt-in by enabling this option. This flag is
+  available since 1.7 and will be removed in moor 2.0, where this flag will always
+  be enabled.
+* `write_from_json_string_constructor`: boolean. Adds a `.fromJsonString` factory
+   constructor to generated data classes. By default, we only write a `.fromJson`
+   constructor that takes a `Map<String, dynamic>`.