add cert-revcheck

cmd/cert-revcheck/README.txt

@@ -0,0 +1,36 @@
+cert-revcheck: check certificate expiry and revocation
+-----------------------------------------------------
+
+Description
+  cert-revcheck accepts a list of certificate files (PEM or DER) or
+  site addresses (host[:port]) and checks whether the leaf certificate
+  is expired or revoked. Revocation checks use CRL and OCSP via the
+  certlib/revoke package.
+
+Usage
+  cert-revcheck [options] <target> [<target>...]
+
+Options
+  -hardfail       treat revocation check failures as fatal (default: false)
+  -timeout dur    HTTP/OCSP/CRL timeout for network operations (default: 10s)
+  -v              verbose output
+
+Targets
+  - File paths to certificates in PEM or DER format.
+  - Site addresses in the form host or host:port. If no port is
+    provided, 443 is assumed.
+
+Examples
+  # Check a PEM file
+  cert-revcheck ./server.pem
+
+  # Check a DER (single) certificate
+  cert-revcheck ./server.der
+
+  # Check a live site (leaf certificate)
+  cert-revcheck example.com:443
+
+Notes
+  - For sites, only the leaf certificate is checked.
+  - When -hardfail is set, network issues during OCSP/CRL fetch will
+    cause the check to fail (treated as revoked).