Warn that NULL SRV records are unsupported in DNSimple provider

DNSimple does not handle NULL SRV records correctly (either via
their web interface or API).  Flag to end user if attempted.

Issue noted with DNSimple support 2020-12-09

tests/test_octodns_provider_dnsimple.py

@@ -137,7 +137,7 @@ class TestDnsimpleProvider(TestCase):
         plan = provider.plan(self.expected)
 
         # No root NS, no ignored, no excluded
-        n = len(self.expected.records) - 4
+        n = len(self.expected.records) - 6
         self.assertEquals(n, len(plan.changes))
         self.assertEquals(n, provider.apply(plan))
         self.assertFalse(plan.exists)