docs: add missing DKIM signing options and vmc date option

- Add expires and headerList options to DKIM signing documentation
- Add --date option to vmc CLI command documentation

Author: andris9

README.md

@@ -163,6 +163,8 @@ const signResult = await dkimSign(message, options);
     - **canonicalization** (`string`): Canonicalization method. Defaults to `'relaxed/relaxed'`.
     - **algorithm** (`string`): Signing and hashing algorithm. Defaults to `'rsa-sha256'`.
     - **signTime** (`Date`): Signing time. Defaults to current time.
+    - **expires** (`Date`): Signature expiration time (`x=` tag). Optional.
+    - **headerList** (`Array`): List of header field names to sign. Optional; uses default set if not specified.
     - **signatureData** (`Array`): Array of signature objects. Each object may contain:
         - **signingDomain** (`string`): DKIM key domain name.
         - **selector** (`string`): DKIM key selector.

cli.md

@@ -268,6 +268,7 @@ mailauth vmc [options]
 - `--authority <url>`, `-a <url>`: URL of the VMC resource.
 - `--authorityPath <path>`, `-p <path>`: Path to a local VMC file, used to avoid network requests.
 - `--domain <domain>`, `-d <domain>`: Sender domain to validate against the certificate.
+- `--date <timestamp>`, `-t <timestamp>`: ISO-formatted timestamp for certificate expiration checks. Defaults to the current time.
 
 #### Example