docs: organize the directory, #656

docs/specification/validators.rst

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+Validators
+==========
+
+Validators are responsible for committing new blocks in the blockchain.
+These validators participate in the consensus protocol by broadcasting
+*votes* which contain cryptographic signatures signed by each
+validator's public key.
+
+Some Proof-of-Stake consensus algorithms aim to create a "completely"
+decentralized system where all stakeholders (even those who are not
+always available online) participate in the committing of blocks.
+Tendermint has a different approach to block creation. Validators are
+expected to be online, and the set of validators is permissioned/curated
+by some external process. Proof-of-stake is not required, but can be
+implemented on top of Tendermint consensus. That is, validators may be
+required to post collateral on-chain, off-chain, or may not be required
+to post any collateral at all.
+
+Validators have a cryptographic key-pair and an associated amount of
+"voting power". Voting power need not be the same.
+
+Becoming a Validator
+--------------------
+
+There are two ways to become validator.
+
+1. They can be pre-established in the `genesis
+   state <./genesis.html>`__
+2. The `ABCI app responds to the EndBlock
+   message <https://github.com/tendermint/abci>`__ with changes to the
+   existing validator set.
+
+Committing a Block
+------------------
+
+*+2/3 is short for "more than 2/3"*
+
+A block is committed when +2/3 of the validator set sign `precommit
+votes <./block-structure.html#vote>`__ for that block at the same
+``round``. The +2/3 set of precommit votes is
+called a `*commit* <./block-structure.html#commit>`__. While any
++2/3 set of precommits for the same block at the same height&round can
+serve as validation, the canonical commit is included in the next block
+(see `LastCommit <./block-structure.html>`__).