add doc/check-certificates.md

doc/check-certificates.md

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+Renew certificates and notify on expiration
+===========================================
+
+[◀ Go back to main README](../README.md)
+
+Description
+-----------
+
+This script tries to download and renew certificates, then notifies about
+certificates that are still about to expire.
+
+Requirements and installation
+-----------------------------
+
+Just install the script:
+
+    $ScriptInstallUpdate check-certificates;
+
+Configuration
+-------------
+
+The expiry notifications just require notification settings for e-mail and
+telegram.
+
+For automatic download and renewal of certificates you need configuration
+in `global-config-overlay`, these are the parameters:
+
+* `CertRenewPass`: an array of passphrases to try
+* `CertRenewUrl`: the url to download certificates from
+
+Certificates on the web server should be named `CN.pem` (`PEM` format) or
+`CN.p12` (`PKCS#12` format).
+
+Usage and invocation
+--------------------
+
+Just run the script:
+
+    / system script run check-certificates;
+
+... or create a scheduler for periodic execution:
+
+    / system scheduler add interval=1d name=check-certificates on-event="/ system script run check-certificates;" start-time=startup;
+
+See also
+--------
+
+* [Renew locally issued certificates](certificate-renew-issued.md)
+
+---
+[◀ Go back to main README](../README.md)  
+[▲ Go back to top](#top)