Add spec for the minimum fee

Author: Seulgi Kim

spec/Minimum-Fee.md

@@ -0,0 +1,16 @@
+Transactions have a different minimum fee per type.
+
+The current minimum fee of transaction is defined as:
+
+| Transaction         | Minimum fee([CCC](CodeChain-Coin.md)) |
+|---------------------|-----------------:|
+| Pay                 | 100              |
+| SetRegularKey       | 10,000           |
+| Store               | 5,000            |
+| Remove              | 5,000            |
+| MintAsset           | 100,000          |
+| TransferAsset       | 100              |
+| ChangeAssetScheme   | 100,000          |
+| IncreaseAssetSupply | 100,000          |
+| WrapCCC             | 100,000          |
+| UnwrapCCC           | 100              |

spec/Transaction.md

@@ -2,6 +2,8 @@
 
 Only a platform account can generate a transaction.
 The transaction generator pays the transaction fees.
+Transactions that cost less than the [minimum fee](Minimum-Fee.md) are rejected.
+The minumum fee is different for each transaction type.
 
 The seq must be identical with the payer’s account seq.
 The account seq will be increased by 1 after a transaction is added to the block.