add changelog

.changelog/2794.breaking.md

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+go/staking: Three-way fee split
+
+We should give more to the previous block proposer, which is the block
+that first ran the transactions that paid the fees in the
+`LastBlockFees`.
+Currently they only get paid as a voter.
+
+See
+[oasis-core#2794](https://github.com/oasislabs/oasis-core/pull/2794)
+for a description of the new fee split.
+
+Instructions for genesis document maintainers:
+
+1. Rename `fee_split_vote` to `fee_split_weight_vote` and
+   `fee_split_propose` to `fee_split_weight_next_propose` and
+   add `fee_split_weight_propose` in `.staking.params`.