fix: add the missed file "extend-expect.js" to package.json (#28)

Author: antsmartian

Diff for: package.json

@@ -19,7 +19,8 @@
   },
   "files": [
     "dist",
-    "typings"
+    "typings",
+    "extend-expect.js"
   ],
   "keywords": [],
   "author": "Kent C. Dodds <[email protected]> (http://kentcdodds.com/)",
@@ -62,4 +63,4 @@
     "url": "https://github.com/kentcdodds/react-testing-library/issues"
   },
   "homepage": "https://github.com/kentcdodds/react-testing-library#readme"
-}
+}