Start writing mempool specification

Include overview and configuration options.

mempool/docs/README.md

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+# Mempool Specification
+
+This package contains documents specifying the functionality
+of the mempool module.
+
+Components:
+
+* [Config](./config.md) - how to configure it
+* [Functionality](./functionality.md) - high-level description of the functionality it provides
+* [External Messages](./messages.md) - The messages we accept over p2p and rpc interfaces
+* [Local Services](./services.md) - Interfaces with consensus and abci services

mempool/docs/config.md

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+# Mempool Configuration
+
+Here we describe configuration options around mempool.
+For the purposes of this document, they are described
+as command-line flags, but they can also be passed in as
+environmental variables or in the config.toml file. The
+following are all equivalent:
+
+Flag: `--mempool.recheck_empty=false`
+
+Environment: `TM_MEMPOOL_RECHECK_EMPTY=false`
+
+Config:
+```
+[mempool]
+recheck_empty = false
+```
+
+
+## Recheck
+
+`--mempool.recheck=false` (default: true)
+
+`--mempool.recheck_empty=false` (default: true)
+
+Recheck determines if the mempool rechecks all pending
+transactions after a block was committed. Once a block
+is committed, the mempool removes all valid transactions
+that were successfully included in the block.
+
+If `recheck` is true, then it will rerun CheckTx on
+all remaining transactions with the new block state.
+
+If the block contained no transactions, it will skip the
+recheck unless `recheck_empty` is true.
+
+## Broadcast
+
+`--mempool.broadcast=false` (default: true)
+
+Determines whether this node gossips any valid transactions
+that arrive in mempool. Default is to gossip anything that
+passes checktx. If this is disabled, transactions are not
+gossiped, but instead stored locally and added to the next
+block this node is the proposer.
+
+## WalDir
+
+`--mempool.wal_dir=/tmp/gaia/mempool.wal` (default: $TM_HOME/data/mempool.wal)
+
+This defines the directory where mempool writes the write-ahead
+logs. These files can be used to reload unbroadcasted
+transactions if the node crashes.
+
+If the directory passed in is an absolute path, the wal file is
+created there. If the directory is a relative path, the path is
+appended to home directory of the tendermint process to
+generate an absolute path to the wal directory
+(default `$HOME/.tendermint` or set via `TM_HOME` or `--home``)