docs: add explanation about swiftpm2nix

Author: i10416

README.md

@@ -29,6 +29,8 @@ This flake provides `protoc-gen-swift` and `protoc-gen-grpc-swift` packages for
 
 ## Develop
 
+### build
+
 `nix build .` builds `protoc-gen-swift` by default. Run `nix build .#protoc-gen-swift` and `nix build .#protoc-gen-grpc-swift` to build each package.
 
 Derivations under `protoc-gen-swift/` and `protoc-gen-grpc-swift/` can be built by `nix-build` command. They have default grpc-swift source, but they are supposed to be used for debug purpose.
@@ -38,3 +40,18 @@ cd protoc-gen-swift
 nix-build .
 ```
 
+### swiftpm2nix
+
+Files under grpc-swift-src/swiftpm2nix are generated by the following steps.
+
+```sh
+git clone [email protected]:grpc/grpc-swift.git -b 1.23.0 grpc-swift-src
+cd grpc-swift-src
+nix-shell -p swift swiftpm swiftpm2nix
+swift package resolve
+swiftpm2nix
+# Move files under grpc-swift-src/nix to grpc-swift/swiftpm2nix.
+```
+
+NOTE:
+These steps are taken from https://git.ri.se/lars.rasmusson/nixpkgs/-/blob/master/doc/languages-frameworks/swift.section.md.