Add is_signature_valid method on CertificateRevocationList (#3849)

diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index eb95a62..ec6a1d0 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -369,6 +369,11 @@
 * ``crl_ian_aia_aki.pem`` - Contains a CRL with ``IssuerAlternativeName``,
   ``AuthorityInformationAccess``, ``AuthorityKeyIdentifier`` and ``CRLNumber``
   extensions.
+* ``valid_signature.pem`` - Contains a CRL with the public key which was used
+  to generate it.
+* ``invalid_signature.pem`` - Contains a CRL with the last signature byte
+  incremented by 1 to produce an invalid signature, and the public key which
+  was used to generate it.
 
 Hashes
 ~~~~~~
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 8b97611..47f7625 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -563,6 +563,12 @@
             over the network and used as part of a certificate verification
             process.
 
+    .. method:: is_signature_valid(public_key)
+
+        .. versionadded:: 2.1
+
+        Returns True if the CRL signature is correct for given public key,
+        False otherwise.
 
 X.509 Certificate Builder
 ~~~~~~~~~~~~~~~~~~~~~~~~~