add changelog and a warning for is_signature_valid on crl (#3861)

* add changelog and a warning

* document this properly
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 7e6b907..db101a8 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -32,6 +32,8 @@
   :attr:`~cryptography.x509.RFC822Name.value` attribute was deprecated, users
   should use :attr:`~cryptography.x509.RFC822Name.bytes_value` to access the
   raw value.
+* Added :meth:`~cryptography.x509.CertificateRevocationList.is_signature_valid`
+  to :class:`~cryptography.x509.CertificateRevocationList`.
 
 
 .. _v2-0-3:
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 47f7625..5e1c95c 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -567,6 +567,12 @@
 
         .. versionadded:: 2.1
 
+        .. warning::
+
+            Checking the validity of the signature on the CRL is insufficient
+            to know if the CRL should be trusted. More details are available
+            in :rfc:`5280`.
+
         Returns True if the CRL signature is correct for given public key,
         False otherwise.