Added consensus doc stub

docs/source/consensus.md

@@ -0,0 +1,27 @@
+# BigchainDB Consensus Plugins
+TODO: Write this section
+
+A quick example of a plugin that adds nonsense rules:
+
+```python
+from bigchaindb.consensus import BaseConsensusRules
+
+class SillyConsensusRules(BaseConsensusRules):
+
+    @staticmethod
+    def validate_transaction(bigchain, transaction):
+        transaction = super().validate_transaction(bigchain, transaction)
+        # I only like transactions whose timestamps are even.
+        if transaction['transaction']['timestamp'] % 2 != 0:
+            raise StandardError("Odd... very odd indeed.")
+        return transaction
+
+    @staticmethod
+    def validate_block(bigchain, block):
+        block = super().validate_block(bigchain, block)
+        # I don't trust Alice, I think she's shady.
+        if block['block']['node_pubkey'] == '<ALICE_PUBKEY>':
+            raise StandardError("Alice is shady, everybody ignore her blocks!")
+
+       return block
+```