feat(nebula-build-core): add docs and help info (#659)

commands/build/README.md

@@ -23,4 +23,5 @@ Options:
   --sourcemap, -m  Generate source map                 [boolean] [default: true]
   --mode           Explicitly set mode
                                  [string] [choices: "production", "development"]
+  --core           Set a core build target            [string] [default: "core"]
 ```

commands/build/lib/init-config.js

@@ -28,6 +28,11 @@ const options = {
     type: 'string',
     default: undefined,
   },
+  core: {
+    description: 'Set a core build target',
+    type: 'string',
+    default: 'core',
+  },
 };
 
 module.exports = (yargs) => {