Update README.md with a link to Astar Network Audit Reports repo

[andabak]

Pull Request Summary
Update README.md with a link to Astar Network Audit Reports repo

Check list

• added or updated unit tests
• updated Astar official documentation
• added OnRuntimeUpgrade hook for precompile revert code registration
• added benchmarks & weights for any modified runtime logics.

[github-actions]

Package	Line Rate	Branch Rate	Health
pallets/block-reward/src	85%	0%	✔
primitives/src	65%	0%	➖
precompiles/xvm/src	75%	0%	➖
pallets/unified-accounts/src	84%	0%	✔
precompiles/xcm/src	75%	0%	➖
pallets/dapps-staking/src/pallet	85%	0%	✔
pallets/xvm/src	40%	0%	❌
pallets/dynamic-evm-base-fee/src	81%	0%	✔
precompiles/unified-accounts/src	100%	0%	✔
pallets/dapps-staking/src	81%	0%	✔
chain-extensions/types/unified-accounts/src	0%	0%	❌
chain-extensions/types/xvm/src	0%	0%	❌
chain-extensions/types/dapps-staking/src	0%	0%	❌
precompiles/substrate-ecdsa/src	78%	0%	➖
precompiles/utils/src/testing	38%	0%	❌
primitives/src/xcm	66%	0%	➖
pallets/xc-asset-config/src	53%	0%	➖
precompiles/dapps-staking/src	93%	0%	✔
precompiles/utils/src	55%	0%	➖
chain-extensions/types/assets/src	0%	0%	❌
precompiles/utils/macro/src	0%	0%	❌
pallets/block-rewards-hybrid/src	87%	0%	✔
chain-extensions/dapps-staking/src	0%	0%	❌
chain-extensions/unified-accounts/src	0%	0%	❌
chain-extensions/xvm/src	0%	0%	❌
pallets/ethereum-checked/src	48%	0%	❌
pallets/contracts-migration/src	0%	0%	❌
chain-extensions/pallet-assets/src	0%	0%	❌
pallets/collator-selection/src	69%	0%	➖
precompiles/assets-erc20/src	77%	0%	➖
precompiles/sr25519/src	79%	0%	➖
Summary	57% (2303 / 4040)	0% (0 / 0)	➖

Minimum allowed line rate is 50%