Comment on usage of null in config files.

Author: mkachk

docsource/content.md

@@ -82,7 +82,7 @@ One of the following modes must be supplied as the **first (and only) argument**
 
 ### 1. `config\config.json`
 
-> See `stock-config.json` for a complete example.
+> See `stock-config.json` for a complete example. Please input `null` to set a value is empty.
 
 - **`digicertApiKey`** - CertCentral API key. Use a key created with **API key restrictions = None**.
 - **`keyfactorDomainAndUser`** - e.g., `DOMAIN\\Username`. User must be permitted to use the Keyfactor API, create/edit metadata fields, and edit certificates.
@@ -221,3 +221,4 @@ Logging uses **NLog** and writes to a local `logs/` folder.
 - **Custom fields with options in DigiCert** - The DigiCert API cannot create dropdown/option fields; create these manually in CertCentral.
 
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+