Add documentation on Validator Sets

docs/protocol/Validator Sets.md

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+# Validator Sets
+
+Validator Sets are defined at the protocol level, with the following parameters:
+
+- `index` (VS):        Validator set index, a global key atomically increasing
+from 0.
+- `bond`  (Amount):    Amount of bond per key-share of this validator set.
+- `coins` (Vec<Coin>): Coins managed by this validator set.
+
+At launch, there will solely be validator set 0, managing Bitcoin, Ethereum,
+USDC, DAI, and Monero.
+
+### Multisig Management
+
+Every validator set is expected to form a multisig per curve required by its
+coins. This multisig is secure to hold funds up to 67% of the validator set's
+bond value.
+
+### Participation in the BFT process
+
+All validator sets participate in the BFT process. Specifically, a block
+containing `Oraclization`s for a coin must be approved by the BFT majority of
+the validator set responsible for it, along with the BFT majority of the network
+by bond.
+
+At this time, `Oraclization`s for a coin are only expected to be included when a
+validator from the validator set managing the coin is the producer of the block
+in question.