Checkpoint block validation docs

docs-gh-pages/checkpoint-block-validation.md

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+# CheckpointBlock Validation
+
+Each transaction sent to cluster needs to be validated before proposing it to block/consensus.
+Part of code responsible for validation is in `TransactionValidator.scala`.
+
+## Duplicated transaction validation
+Duplicated transaction can affect wallet balances so block which contains transaction with specific `hash` twice
+is incorrect and needs to be rejected. Method `validateDuplicatedTransactions` returns `DuplicatedTransaction`
+validation result if there are any duplicated transactions.
+
+![alt text](img/checkpoint-block-validation/duplicated-transaction.jpg "Duplicated transaction")
+
+## Signature validation and hash integrity
+Each block has its unique signature which is created by signing block by private key.
+Thanks to that we know that block was created for sure by known signer.
+Each block has also unique `hash` which is generated from the content of block so if someone malformed the content
+of block - the hash should be different. If hash is different or does not match the content of block then such block
+is rejected because is malformed.
+
+## Balance validation
+Balance validation checks if each of addresses could make transactions included in the block.
+Of course there is a transaction validator, but some things can't be validated on single transaction level
+so we need to validate whole block and all the transactions inside for specific address.
+
+### Insufficient balance validation (for regular wallets)
+First of all we must check if specific wallet has enough tokens to make all the transactions included in block.
+That being said balance left after executing transactions should be `>= 0` (so can't be negative).
+To do that `validateSourceAddressBalances` sums sent tokens for address and compares it with current balance.
+If sum is bigger than current balance then address has `InsufficientBalance` and block is incorrect because
+wallet has not enough tokens to send all the transactions from block.
+
+### Staking amount validation (for node wallets)
+Node wallets have slightly different rules of validating balance left after executing transactions.
+There is a minimal amount of tokens which needs to stay untouched called **staking amount** which is currently
+set to `250000` tokens (can be configured via `constellation.staking-amount`). That being said similarly to
+Insufficient balance validation, after executing all the transactions for node wallet there must be a **staking amount**
+of tokens left in the wallet. In case of failure it returns `InsufficientBalance`.
+
+## Transaction integrity
+Checking integrity of transactions in the block is just about validating each transaction separately in the same
+way as described in [Transaction Validation](transaction-validation.md) docs.
+