Add a short description on each library in `README.md`

[0xs34n]

See Solmate:

auth
├─ Auth — "Flexible and updatable auth pattern"
├─ authorities
│  ├─ RolesAuthority — "Role based Authority that supports up to 256 roles"
│  ├─ MultiRolesAuthority — "Flexible and target agnostic role based Authority"
tokens
├─ WETH — "Minimalist and modern Wrapped Ether implementation"
├─ ERC20 — "Modern and gas efficient ERC20 + EIP-2612 implementation"
├─ ERC721 — "Modern, minimalist, and gas efficient ERC721 implementation"
├─ ERC1155 — "Minimalist and gas efficient standard ERC1155 implementation"
utils
├─ SSTORE2 - "Library for cheaper reads and writes to persistent storage"
├─ CREATE3 — "Deploy to deterministic addresses without an initcode factor"
├─ SafeCastLib - "Safe unsigned integer casting lib that reverts on overflow"
├─ ReentrancyGuard — "Gas optimized reentrancy protection for smart contracts"
├─ FixedPointMathLib — "Arithmetic library with operations for fixed-point numbers"
├─ Bytes32AddressLib — "Library for converting between addresses and bytes32 values"
├─ SafeTransferLib — "Safe ERC20/ETH transfer lib that handles missing return values"