update README to document API_SECRET_FILE environment variable

Author: swebster

README.md

@@ -238,7 +238,7 @@ To learn more about the Nightscout API, visit https://YOUR-SITE.com/api-docs/ or
 ### Required
 
   * `MONGODB_URI` - The connection string for your Mongo database. Something like `mongodb://sally:sallypass@ds099999.mongolab.com:99999/nightscout`.
-  * `API_SECRET` - A secret passphrase that must be at least 12 characters long.
+  * `API_SECRET` - A secret passphrase that must be at least 12 characters long. Alternatively, if `API_SECRET_FILE` is defined, the secret passphrase will be read from the specified file.
   * `MONGODB_COLLECTION` (`entries`) - The Mongo collection where CGM entries are stored.
   * `DISPLAY_UNITS` (`mg/dl`) - Options are `mg/dl` or `mmol/L` (or just `mmol`).  Setting to `mmol/L` puts the entire server into `mmol/L` mode by default, no further settings needed.