Add support for readme autogeneration for content package

spec/changelog.yml

@@ -26,6 +26,9 @@
     - description: Add geo_shape as a valid field type for integrations.
       type: enhancement
       link: https://github.com/elastic/package-spec/pull/1132
+    - description: Allow _dev/build/docs folder in content packages to support README auto-generation.
+      type: enhancement
+      link: https://github.com/elastic/package-spec/pull/1144
 - version: 3.6.0
   changes:
     - description: Add pipeline tag validations.

spec/content/_dev/spec.yml

@@ -2,6 +2,11 @@ spec:
   additionalContents: false
   developmentFolder: true
   contents:
+    - description: Folder containing resources related to building the package.
+      type: folder
+      name: build
+      required: false
+      $ref: "../../integration/_dev/build/spec.yml"
     - description: Folder containing shared files, including experimental dashboards-as-YML files for content-only packages.
       type: folder
       name: shared

test/packages/good_content_with_dev/_dev/build/docs/README.md

@@ -0,0 +1 @@
+# Reference package of content type