chore: improve docs in README (#6)

* add interfaces from spec

* ^0.8.13

* notes docs

Author: zerosnacks

README.md

@@ -2,13 +2,31 @@
 
 Tempo Standard Library is a collection of helpful Tempo specific contracts and libraries for use with [Foundry](https://github.com/foundry-rs/foundry).
 
-## Installation
+### Installation
 
 ```
 forge install tempoxyz/tempo-std
 ```
 
-## License
+### Contracts
+
+```
+src/
+├── interfaces
+│   ├── IStablecoinExchange.sol
+│   ├── ITIP20Factory.sol
+│   ├── ITIP20.sol
+│   ├── ITIP403Registry.sol
+│   └── ITIP4217Registry.sol
+├── StdContracts.sol
+└── StdPrecompiles.sol
+```
+
+### Support
+
+The current minimum supported Solidity version is `0.8.13`.
+
+### License
 
 Licensed under either of [Apache License](./LICENSE-APACHE), Version
 2.0 or [MIT License](./LICENSE-MIT) at your option.