add upgrading guides

UPGRADING.md

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+# Upgrading Tendermint Core
+
+This guide provides steps to be followed when you upgrade your applications to
+a newer version of Tendermint Core.
+
+## Upgrading from 0.23.0 to 0.24.0
+
+New 0.24.0 release contains a lot of changes to the state and types. It's not
+compatible to the old versions.
+
+To reset the state do:
+
+```
+$ tendermint unsafe_reset_all
+```
+
+### Config changes
+
+`p2p.max_num_peers` was removed in favor of `p2p.max_num_inbound_peers` and
+`p2p.max_num_outbound_peers`. 
+
+```
+# Maximum number of inbound peers
+max_num_inbound_peers = 40
+
+# Maximum number of outbound peers to connect to, excluding persistent peers
+max_num_outbound_peers = 10
+```
+
+As you can see, the default ratio of inbound/outbound peers is 4/1. The reason
+as we want it to be easier for new nodes to connect to the network. You can
+tweak these parameters to alter the network topology.