DOCS-4246: recover invalid replset config

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Signed-off-by: Allison Moore <[email protected]>

Author: schmalliso

source/release-notes/2.8-compatibility.txt

@@ -143,6 +143,22 @@ particular note are the following alterations:
 
 - Disallows ``getLastErrorDefaults: 0`` in a config.
 
+.. _recover-from-invalid-config:
+
+Recover a Replica Set from an Invalid Configuration
+```````````````````````````````````````````````````
+
+In MongoDB version 2.8, :program:`mongod` will fail to start if it has
+an invalid replica set configuration stored in the
+:data:`local.system.replset` collection. Due to MongoDB 2.8's strict
+validation of replica set configuration objects, a configuration that
+was valid in MongoDB 2.6 may not be valid in MongoDB 2.8.
+
+If your replica set configuration was valid in a previous version of
+MongoDB and fails to start in MongoDB 2.8, :doc:`downgrade to MongoDB
+2.6 </release-notes/2.8-downgrade>`, and reconfigure your replica set to
+fulfill the new requirements. Then, you can re-upgrade to MongoDB 2.8.
+
 Remove ``local.slaves`` Collection
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