Make the C# package README user-facing

payjoin-ffi/csharp/CONTRIBUTING.md

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+# Contributing to the Payjoin C# Bindings
+
+C# bindings for the [Payjoin Dev Kit](https://payjoindevkit.org/), generated from
+`payjoin-ffi` with UniFFI. This document covers building the bindings from
+source, running the tests, and producing the NuGet package. For using the
+published package, see [`README.md`](./README.md).
+
+## Development
+
+With nix, the C# development shell provides the Rust toolchain and .NET 10 SDK:
+
+```shell
+nix develop .#csharp -c bash payjoin-ffi/csharp/contrib/test.sh
+```
+
+Without nix, a Rust toolchain (MSRV: 1.85.0 for this repository) and the .NET 10
+SDK are required:
+
+```shell
+git clone https://github.com/payjoin/rust-payjoin.git
+cd rust-payjoin/payjoin-ffi/csharp
+
+bash ./scripts/generate_bindings.sh
+dotnet build Payjoin.Tests.csproj
+dotnet test Payjoin.Tests.csproj
+```
+
+### Windows
+
+```powershell
+git clone https://github.com/payjoin/rust-payjoin.git
+cd rust-payjoin/payjoin-ffi/csharp
+
+powershell -ExecutionPolicy Bypass -File .\scripts\generate_bindings.ps1
+dotnet build Payjoin.Tests.csproj
+dotnet test Payjoin.Tests.csproj
+```
+
+Generation uses the Cargo-managed C# generator pinned in `payjoin-ffi/Cargo.toml`.
+By default, development generation enables `_test-utils` to keep parity with the
+test suite. For production bindings, set `PAYJOIN_FFI_FEATURES` to an empty value
+(bash) or pass `-ProductionBindings` (PowerShell — Windows cannot represent an
+empty environment variable, so the switch is the only reliable signal there).
+
+## Packaging
+
+Build the release native asset for the current host RID (production features are
+the default when `PAYJOIN_FFI_FEATURES` is not set; this step does not regenerate
+the C# bindings):
+
+```shell
+bash ./scripts/build_nuget_native.sh
+```
+
+Any supported RID can also be cross-compiled from a Linux host, which is how CI
+builds every native asset (`pip install -r scripts/cross-requirements.txt` for
+the version- and hash-pinned toolchain CI uses, and `rustup target add` the
+matching triple first):
+
+```shell
+PAYJOIN_FFI_CROSS=1 PAYJOIN_FFI_RID=osx-arm64 bash ./scripts/build_nuget_native.sh
+```
+
+On Windows:
+
+```powershell
+powershell -ExecutionPolicy Bypass -File .\scripts\build_nuget_native.ps1
+```
+
+To pack, gather every supported RID under `artifacts/runtimes/{rid}/native/`,
+generate production bindings, and run:
+
+```shell
+PAYJOIN_FFI_FEATURES= PAYJOIN_FFI_PROFILE=release bash ./scripts/generate_bindings.sh
+dotnet pack Payjoin.csproj --configuration Release --output artifacts/packages
+```
+
+On Windows:
+
+```powershell
+$env:PAYJOIN_FFI_PROFILE = "release"
+powershell -ExecutionPolicy Bypass -File .\scripts\generate_bindings.ps1 -ProductionBindings
+dotnet pack Payjoin.csproj --configuration Release --output artifacts/packages
+```
+
+Validate the package in a clean sample app (`auto` derives the version from the
+packed artifact; an explicit version is also accepted):
+
+```shell
+bash ./scripts/smoke_nuget_package.sh artifacts/packages auto linux-x64
+```
+
+CI performs the package build from release native assets and runs the smoke test
+on each supported RID before publishing should be considered. The maintainer
+release and publish workflow is documented in
+[`RELEASING.md`](https://github.com/payjoin/rust-payjoin/blob/master/payjoin-ffi/csharp/RELEASING.md).