message: Add mempool.dat to doc/files.md
diff: * db.log: wallet database log file
  * debug.log: contains debug information and general logging generated by bitcoind or bitcoin-qt
  * fee_estimates.dat: stores statistics used to estimate minimum transaction fees and priorities required for confirmation; since 0.10.0
  +* mempool.dat: dump of the mempool's transactions; since 1.5.0.1.
  * peers.dat: peer IP address database (custom format); since 0.7.0
  * wallet.dat: personal wallet (BDB) with keys and transactions
  * .cookie: session RPC authentication cookie (written at start when cookie authentication is used, deleted on shutdown): since 0.12.0
  