docs: Extend mopro-ffi with guide for extending custom functions

guide-for-extending-ffi.md

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+# Guide for Extending Custom Functions Through Mopro
+
+This guide will show you how to embed your own function into `mopro-ffi`, successfully generate either Swift or Kotlin bindings, and create a corresponding static library (e.g. xcframework if your target is iOS).
+
+## Prerequisites
+
+Before getting started, ensure you have:
+1. Created your own function logic in Rust.
+2. Forked the `mopro` repository.
+
+## Import Your Crates in `mopro-ffi/src/lib.rs`
+
+To generate Rust scaffolding, import your crates inside the `lib.rs` file in `mopro-ffi`. We recommend creating a replicate function in `lib.rs` that calls your functions from the imported crates. Then, create the same function inside the macro to get the relevant functions into scope and call FFI.
+
+For example:
+
+```rust
+// mopro-ffi/src/lib.rs
+
+use your_crates;
+
+pub fn your_function(a: u32, b: u32) -> Result<bool, MoproError> {
+    // Function logic
+    your_crates::some_function()
+}
+
+#[macro_export]
+macro_rules! app {
+    () => {
+        // Remember to import the struct here if you are using a custom one
+        use mopro_ffi::{BenchmarkResult, GenerateProofResult, MoproError, ProofCalldata, G1, G2};
+        use std::collections::HashMap;
+
+        fn your_function(a: u32, b: u32) -> Result<bool, MoproError> {
+            mopro_ffi::your_function(a, b)
+        }
+
+        uniffi::include_scaffolding!("mopro");
+    };
+}
+```
+
+### Using Feature Flags
+
+If you are using a feature flag, handle the situation when the flag is not activated.
+
+For example:
+
+```rust
+// mopro-ffi/src/lib.rs
+
+use your_crates;
+
+#[cfg(feature = "your-feature")]
+pub fn your_function(a: u32, b: u32) -> Result<bool, MoproError> {
+    // Function logic
+    your_crates::some_function()
+}
+
+#[cfg(not(feature = "your-feature"))]
+pub fn your_function(_: u32, _: u32) -> Result<bool, MoproError> {
+    Err(MoproError::YourCustomError("something went wrong".to_string()))
+}
+```
+
+## Extend the UDL File for Custom Functions
+
+Modify `mopro-ffi/src/mopro.udl` to define your functions and variable types. Follow the [uniffi documentation](https://mozilla.github.io/uniffi-rs/0.28/udl/builtin_types.html) for more details.
+
+For example:
+
+```udl
+// mopro-ffi/src/mopro.udl
+
+namespace mopro {
+  // ... other definitions
+
+  [Throws=MoproError]
+  boolean your_function(u32 a, u32 b);
+};
+
+dictionary YourStruct {
+  u32 a;
+  u32 b;
+};
+```
+
+## Build Bindings for Swift and Kotlin
+
+First, ensure the library name in `mopro-ffi/Cargo.toml` is "mopro_bindings". This guarantees the configuration is correctly set up for binding generation.
+
+```toml
+# mopro-ffi/Cargo.toml
+
+[lib]
+name = "mopro_bindings"  # Make sure the lib name is correct
+```
+
+Next, run the `mopro-ffi/build_bindings.sh` script to generate bindings for your custom functions. Remember to re-run it every time you modify the UDL file and push the resulting output to your repo.
+
+## Access Custom Mopro Repo from Your App
+
+Congratulations! Your custom functions have been successfully added to `mopro`. Now, let's integrate `mopro` into your app project. If you don't have an app project yet, we recommend forking one from [mopro-app](https://github.com/chancehudson/mopro-app).
+
+First, follow the steps in [Mopro/Rust-setup](https://zkmopro.org/docs/getting-started/rust-setup) to create a valid setup.
+
+Next, include your forked `mopro` dependencies and [uniffi](https://crates.io/crates/uniffi) in your Rust directory.
+
+```toml
+[package]
+name = "your_app"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["lib", "cdylib", "staticlib"]
+name = "mopro_bindings"  # This library name should not be changed
+
+[[bin]]
+name = "ios"
+
+[features]
+default = ["mopro-ffi/circom"]
+
+[dependencies]
+# ... other dependencies
+mopro-ffi = { git = "https://github.com/zkmopro/mopro.git", branch = "feat/integrate-gpu-acceleration", features = ["gpu-acceleration"] }
+uniffi = { version = "0.28", features = ["cli"] }
+
+[build-dependencies]
+# ... other dependencies
+mopro-ffi = { git = "https://github.com/zkmopro/mopro.git", branch = "feat/integrate-gpu-acceleration", features = ["gpu-acceleration"] }
+uniffi = { version = "0.28", features = ["build"] }
+```
+
+Finally, run the following command in the terminal to build the static library.
+
+```sh
+# CONFIGURATION is either debug or release
+CONFIGURATION=release cargo run --bin ios
+CONFIGURATION=debug cargo run --bin ios
+```
+
+Now, you can open `your-app/ios/your-app.xcodeproj` to check that your custom functions have been successfully bound to your app.