Add an exports field for README

Author: hasparus

packages/theme-ui/package.json

@@ -33,7 +33,8 @@
       },
       "default": "./jsx-dev-runtime/dist/theme-ui-jsx-dev-runtime.cjs.js"
     },
-    "./package.json": "./package.json"
+    "./package.json": "./package.json",
+    "./README.md": "./README.md"
   },
   "types": "dist/theme-ui.cjs.d.ts",
   "sideEffects": false,