op-bindings: add readme

op-bindings/README.md

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+# op-bindings
+
+This package contains built go bindings of the smart contracts. It must be
+updated after any changes to the smart contracts to ensure that the bindings are
+up to date.
+
+The bindings include the bytecode for each contract so that go based tests
+can deploy the contracts. There are also `more` files that include the deployed
+bytecode as well as the storage layout. These are used to dynamically set
+bytecode and storage slots in state.
+
+## Dependencies
+
+- `abigen`
+- `make`
+
+## abigen
+
+The `abigen` tool is part of `go-ethereum` and can be used to build go bindings
+for smart contracts. It can be installed with go using the commands:
+
+```bash
+$ go get -u github.com/ethereum/go-ethereum
+$ cd $GOPATH/src/github.com/ethereum/go-ethereum/
+$ make devtools
+```
+
+The geth docs for `abigen` can be found [here](https://geth.ethereum.org/docs/dapp/native-bindings).