Add README.md for ledger process

src/ripple/module/app/ledger/README.md

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+
+# Ledger Process
+
+## Introduction
+
+## Life Cycle
+
+Every server always has an open ledger. All received new transactions are
+applied to the open ledger. The open ledger can't close until we reach
+a consensus on the previous ledger, and either: there is at least one
+transaction in the open ledger, or the ledger's close time has been reached.
+
+When the open ledger is closed the transactions in the open ledger become
+the initial proposal. Validators will send the proposal (non-validators will
+simply not send the proposal). The open ledger contains the set of transactions
+the server thinks should go into the next ledger.
+
+Once the ledger is closed, servers avalanche to consensus on the candidate
+transaction set and the close time. When consensus is reached, servers build
+a new last closed ledger by starting with the previous last closed ledger and
+applying the consensus transaction set. In the normal case, servers will all
+agree on both the last closed ledger and the consensus transaction set. The
+goal is to give the network the highest chances of arriving at the same
+conclusion on all servers.
+
+At all times during the consensus process the open ledger remains open with the
+same transaction set, and has new transactions applied. It will likely have
+transactions that are also in the new last closed ledger. Valid transactions
+received during the consensus process will only be in the open ledger.
+
+Validators now publish validations of the new last closed ledger. Servers now
+build a new open ledger from the last closed ledger. First, all transactions
+that were candidates in the previous consensus round but didn't make it into
+the consensus set are applied. Next, transactions in the current open ledger
+are applied. This covers transactions received during the previous consensus
+round. This is a "rebase": now that we know the real history, the current open
+ledger is rebased against the last closed ledger.
+
+The purpose of the open ledger is as follows:
+- Forms the basis of the initial proposal during consensus
+- Used to decide if we can reject the transaction without relaying it
+
+## Byzantine Failures
+
+Byzantine failures are resolved as follows. If there is a supermajority ledger,
+then a minority of validators will discover that the consensus round is
+proceeding on a different ledger than they thought. These validators will
+become desynced, and switch to a strategy of trying to acquire the consensus
+ledger.
+
+If there is no majority ledger, then starting on the next consensus round there
+will not be a consensus on the last closed ledger. Another avalanche process
+is started.
+
+## Validators
+
+The only meaningful difference between a validator and a 'regular' server is
+that the validator sends its proposals and validations to the network.
+
+---
+
+# Definitions
+
+## Open Ledger
+
+The open ledger is the ledger that the server applies all new incoming
+transactions to.
+
+## Last Validated Ledger
+
+The most recent ledger that the server is certain will always remain part
+of the permanent, public history.
+
+## Last Closed Ledger
+
+The most recent ledger that the server believes the network reached consensus
+on. Different servers can arrive at a different conclusion about the last
+closed ledger. This is a consequence of Byztanine failure. The purpose of
+validations is to resolve the differences between servers and come to a common
+conclusion about which last closed ledger is authoritative.
+
+## Consensus
+
+A distributed agreement protocol. Ripple uses the consensus process to solve
+the problem of double-spending.
+
+## Validation
+
+A signed statement indicating that it built a particular ledger as a result
+of the consensus process.
+
+## Proposal
+
+A signed statement of which transactions it believes should be included in
+the next consensus ledger.
+
+---
+
+# References
+