Add warning about single-quoting argon password hashes in .env files

README.md

@@ -29,6 +29,7 @@ transbeam is configured with the following environment variables:
   ```bash
   echo -n 'Password: '; head -1 | tr -d '\n' | argon2 $(openssl rand -base64 32) -id -t 2 -m 14 -p 1 -e
   ```
+  (If you're setting this in a `.env` file or in a shell, make sure to single-quote it to prevent text after `$` characters from being interpreted as variables.)
 - `TRANSBEAM_COOKIE_SECRET`: Base64-encoded cryptographic random data
   for encrypting private session cookies. To generate:
   ```bash