Add an FAQ section on including extra sources

docs/faq.md

@@ -38,6 +38,27 @@ targets:
           compiler: dart2js
 ```
 
+## How can I include additional sources in my build?
+
+By default, the `build_runner` package only includes some specifically
+whitelisted directories, derived from the [package layout conventions](
+https://www.dartlang.org/tools/pub/package-layout).
+
+If you have some additional files which you would like to be included as part of
+the build, you will need a custom `build.yaml` file. You will want to modify the
+`sources` field on the `$default` target:
+
+```yaml
+targets:
+  $default:
+    sources:
+      - my_custom_sources/**
+      - lib/**
+      - web/**
+      # Note that it is important to include these in the default target.
+      - pubspec.*
+```
+
 ## Why do Builders need unique outputs?
 
 `build_runner` relies on determining a static build graph before starting a