add terminology to readme

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 # Bytom Node.js SDK
 
+## Terminology
+
+### Keys
+
+Cryptographic keys are the primary authorization mechanism on a blockchain.
+
+### Account
+
+An account is an object in Bytom that tracks ownership of assets on a blockchain. 
+It's defined under one Bytom node created with one or serveral keys.
+
+### Asset
+
+An asset is a type of value that can be issued on a blockchain. All units of
+a given asset are fungible. Units of an asset can be transacted directly
+between parties without the involvement of the issuer.
+
+### Transaction
+
+Blockchain is chain of blocks, while block consists of numbers of transactions.
+
+### Unspent Output(UTXO)
+
+Bytom is UTXO based blockchain. One transaction spend some UTXOs, and produces new UTXOs.
+
+### Balance
+
+Any balance on the blockchain is simply a summation of UTXOs. In one bytomd, balance means
+summation of UTXOs of one account.
+
 ## Usage
 
 ### In your code