message: added workflow diagram
diff: ## Node version
  Need latest version of node 13.7.0. Others version wont allow the `--experimental-wasm-modules` flag to import wasm.
  +
  +
  +## Possible Workflow
  +
  +USER WANT TO SELL FILECOINS
  +
  + /-----------/ /--------------/ /---------/
  + / USER / / EXCHANGE / / NODE /
  + /-----------/ /--------------/ /---------/
  + | | |
  + |------Sell Request----->| |
  + | | key_derive() |
  + |<-----Address-----------| |
  + | | |
  + |------------------Send Transaction----------->|
  + | | |
  + | |<-----Notify---------|
  + | | |
  + |<------Success----------| |
  +
  +
  +USER WANT TO BUY FILECOINS
  +
  + /-----------/ /--------------/ /---------/
  + / USER / / EXCHANGE / / NODE /
  + /-----------/ /--------------/ /---------/
  + | | |
  + |------Buy (address)---->| |
  + | | key_derive() |
  + | | tx_create() |
  + | | sign_transaction() |
  + | | |
  + | |-----Sentd Tx ------>|
  + | | |
  + | | |
  + | |<-----Notify---------|
  + | | |
  + |<------Success----------| |
  