Do not run prettier on markdown files

It seems to introduce some errors on markdown files, like changing a single * to _.

package.json

@@ -21,8 +21,8 @@
     "integrity2": "node buildutils/lib/integrity.js",
     "lint": "yarn && yarn run prettier && yarn run eslint",
     "lint:check": "yarn run prettier:check && yarn run eslint:check",
-    "prettier": "prettier --ignore-path .gitignore --write '**/*{.ts,.tsx,.js,.jsx,.css,.json,.md}'",
-    "prettier:check": "prettier --ignore-path .gitignore --check '**/*{.ts,.tsx,.js,.jsx,.css,.json,.md}'",
+    "prettier": "prettier --ignore-path .gitignore --write '**/*{.ts,.tsx,.js,.jsx,.css,.json}'",
+    "prettier:check": "prettier --ignore-path .gitignore --check '**/*{.ts,.tsx,.js,.jsx,.css,.json}'",
     "sort-package-json": "lerna exec --parallel sort-package-json && sort-package-json",
     "publish": "yarn run clean && yarn run build && lerna publish -m \"Publish npm packages\"",
     "update-dependency": "update-dependency --lerna",