Update with-dotenv example README.md to add troubleshooting informati…

…on (vercel#2995)

examples/with-dotenv/README.md

@@ -29,3 +29,13 @@ now
 This example shows the most basic idea of babel replacement from multiple environment. We have 1 env variable: `TEST` which will be replaced in development env and in production env with different babel plugin. In local development, babel reads .env file and replace process.env.* in your nextjs files. In production env (such as heroku), babel reads the ENV and replace process.env.* in your nextjs files. Thus no more needed to commit your secrets anymore.
 
 Of course, please put .env* in your .gitignore when using this example locally.
+
+## Troubleshooting
+
+### Environment variables not showing on the page
+
+If for some reason the variable is not displayed on the page, try clearing the `babel-loader` cache:
+
+```
+rm -rf ./node_modules/.cache/babel-loader
+```