Readme: suggest SPM as default install method

[apptekstudios]

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This PR adds install instructions for using R.Swift with SPM. This is preferable for those starting new projects and wanting to avoid cocoapods given the now-widespread support for SPM.

I changed it to be the recommended method, however opinions may differ... Please feel free to just add it as another option if that's preferable 😄

[tomlokhorst]

Thanks for this PR!

I haven't tried this yet, but reading the PR, it seems interesting to use SPM in this way. It does seem like quite a few steps to get everything working, but it's nice to not depend on CocoaPods anymore.

I'll have a more detailed look later on.

[apptekstudios]

I opted to keep the instructions very verbose assuming it’s all new to people, but it boils down to very quick steps:

1. Create a Package.swift file
2. Make Run Script
3. Add R.swift.Library and generated file to target

[mac-cain13]

Interesting to use SPM in this way, think we need to write it in a bit more compact way and not make it the default yet.

Also when we add this option think the readme needs a bit of restructuring or foldable parts to keep it readable.

Lets fix those points then merge this.

[quentinfasquel]

@mac-cain13 I am not sure where to ask this.. important R.swift with SPM is fine, some people wrote about how to then use it in your Xcodeproj. However, I don't seem to find anyone who's written of using R.swift "outside an Xcode project". I am developing a swift package "Kit" that internally uses R.swift but I don't have a way to run R.swift for its sources without generating the Xcode project and adding a script phase. Is there any way I could do so without generating the Xcode project ?

[fthdgn]

I don't like the BuildTools step. It is annoying to create folders on root project. This build script may be better.

cd $TEMP_DIR
mkdir RSwiftBinary
cd RSwiftBinary
echo `pwd`
echo "// swift-tools-version:5.1

import PackageDescription
let package = Package(
    name: \"RSwiftBinary\",
    platforms: [.macOS(.v10_11)],
    dependencies: [
        .package(url: \"https://github.com/mac-cain13/R.swift\", from: \"5.4.0\"),
    ]
)
" > Package.swift
/usr/bin/xcrun --sdk macosx swift run -c release rswift generate "$SRCROOT/R.generated.swift"

Another option is that. Content of the Package.swift can be put on a folder on the library project. And we can run the script below. This method allows us to use always use correct R.swift and R.swift.Library library together.

cd "${BUILD_DIR%Build/*}/SourcePackages/checkouts/R.swift.Library/BuildTools/"
/usr/bin/xcrun --sdk macosx swift run -c release rswift generate "$SRCROOT/R.generated.swift"