doc(katana-executor): add README

crates/katana/executor/README.md

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+## katana-executor
+
+This crate provides a set of abstractions for performing transaction executions in Katana. It includes [implementations](./src/implementation/) for the different execution engines that Katana supports:
+
+1. [blockifier](https://github.com/dojoengine/blockifier) by StarkWare
+2. [starknet_in_rust](https://github.com/dojoengine/starknet_in_rust) by LambdaClass. 
+
+They are feature-gated under `blockifier` and `sir` features respectively and  enabled by **default**.
+
+This crate also includes a [*noop*](./src/implementation/noop.rs) implementation for testing purposes.
+
+### Cairo Native support
+
+The [starknet_in_rust](./src/implementation/sir/) executor can be integrated with Cairo Native, which makes the execution of sierra programs possible through native machine code. To use it, you must enable the `native` feature when using this crate as a dependency,
+
+```toml
+[dependencies]
+katana-executor = { .., features = [ "native" ] }
+```
+
+and the following needs to be setup:
+
+LLVM 17 needs to be installed and the `MLIR_SYS_170_PREFIX` and `TABLEGEN_170_PREFIX` environment variable needs to point to said installation. 
+
+In macOS, run
+
+```console
+brew install llvm@17
+```
+
+and export the following environment variables:
+
+```bash
+export MLIR_SYS_170_PREFIX=/opt/homebrew/opt/llvm@17
+export LLVM_SYS_170_PREFIX=/opt/homebrew/opt/llvm@17
+export TABLEGEN_170_PREFIX=/opt/homebrew/opt/llvm@17
+```
+
+and you're set.
+
+