Installation instructions for macOS with Apple Silicon without using homebrew#154

These are my notes to help users to setup yubikey-agent on macOS with Apple Silicon without homebrew: I am skipping the obtain the source code, unpack steps. Also, no instructions provided on how to obtain `go`. To build the package, use ``` CGO_ENABLED="1" GOOS=darwin GOARCH=arm64 go build ``` Copy the binary in place. I picked `/usr/local/bin` and this will be used in below (this can be changed) ``` sudo cp yubikey-agent /usr/local/bin ``` Install for a user: ``` mkdir -p ~/Library/LaunchAgents touch ~/Library/LaunchAgents/launched.yubikey-agent.plist ``` Copy this into the file `launched.yubikey-agent.plist`: ``` <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>Label</key> <string>launched.yubikey-agent</string> <key>ProgramArguments</key> <array> <string>/usr/local/bin/yubikey-agent</string> <string>-l</string> <string>/private/tmp/yubikey-agent.sock</string> </array> <key>RunAtLoad</key> <true/> <key>KeepAlive</key> <true/> </dict> </plist> ``` Enable the service: ``` cd ~/Library/LaunchAgents launchctl bootstrap gui/$UID launched.yubikey-agent.plist launchctl enable gui/$UID/launched.yubikey-agent.plist ``` Then, in `~/.ssh/config`, use: ``` Host * IdentityAgent /private/tmp/yubikey-agent.sock ``` Note: the Apple silicon specific step is the build step. I don't have an Intel-based Mac, so IDK what the step should be. My guess is that the `GOARCH` value would be different. The rest should work on all recent macOS machines, though.