Makefile: add help text to Makefile

https://rosszurowski.com/log/2022/makefiles#self-documenting-makefiles
Signed-off-by: Will Norris <will@tailscale.com>

Makefile

@@ -2,16 +2,13 @@ IMAGE_REPO ?= tailscale/tailscale
 SYNO_ARCH ?= "amd64"
 SYNO_DSM ?= "7"
 
-usage:
-	echo "See Makefile"
-
-vet:
+vet: ## Run go vet
 	./tool/go vet ./...
 
-tidy:
+tidy: ## Run go mod tidy
 	./tool/go mod tidy
 
-updatedeps:
+updatedeps: ## Update depaware deps
 	# depaware (via x/tools/go/packages) shells back to "go", so make sure the "go"
 	# it finds in its $$PATH is the right one.
 	PATH="$$(./tool/go env GOROOT)/bin:$$PATH" ./tool/go run github.com/tailscale/depaware --update \
@@ -19,7 +16,7 @@ updatedeps:
 		tailscale.com/cmd/tailscale \
 		tailscale.com/cmd/derper
 
-depaware:
+depaware: ## Run depaware checks
 	# depaware (via x/tools/go/packages) shells back to "go", so make sure the "go"
 	# it finds in its $$PATH is the right one.
 	PATH="$$(./tool/go env GOROOT)/bin:$$PATH" ./tool/go run github.com/tailscale/depaware --check \
@@ -27,42 +24,42 @@ depaware:
 		tailscale.com/cmd/tailscale \
 		tailscale.com/cmd/derper
 
-buildwindows:
+buildwindows: ## Build tailscale CLI for windows/amd64
 	GOOS=windows GOARCH=amd64 ./tool/go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled
 
-build386:
+build386: ## Build tailscale CLI for linux/386
 	GOOS=linux GOARCH=386 ./tool/go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled
 
-buildlinuxarm:
+buildlinuxarm: ## Build tailscale CLI for linux/arm
 	GOOS=linux GOARCH=arm ./tool/go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled
 
-buildwasm:
+buildwasm: ## Build tailscale CLI for js/wasm
 	GOOS=js GOARCH=wasm ./tool/go install ./cmd/tsconnect/wasm ./cmd/tailscale/cli
 
-buildlinuxloong64:
+buildlinuxloong64: ## Build tailscale CLI for linux/loong64
 	GOOS=linux GOARCH=loong64 ./tool/go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled
 
-buildmultiarchimage:
+buildmultiarchimage: ## Build (and optionally push) multiarch docker image
 	./build_docker.sh
 
-check: staticcheck vet depaware buildwindows build386 buildlinuxarm buildwasm
+check: staticcheck vet depaware buildwindows build386 buildlinuxarm buildwasm ## Perform basic checks and compilation tests
 
-staticcheck:
+staticcheck: ## Run staticcheck.io checks
 	./tool/go run honnef.co/go/tools/cmd/staticcheck -- $$(./tool/go list ./... | grep -v tempfork)
 
-spk:
+spk: ## Build synology package for ${SYNO_ARCH} architecture and ${SYNO_DSM} DSM version
 	PATH="${PWD}/tool:${PATH}" ./tool/go run github.com/tailscale/tailscale-synology@main -o tailscale.spk --source=. --goarch=${SYNO_ARCH} --dsm-version=${SYNO_DSM}
 
-spkall:
+spkall: ## Build synology packages for all architectures and DSM versions
 	mkdir -p spks
 	PATH="${PWD}/tool:${PATH}" ./tool/go run github.com/tailscale/tailscale-synology@main -o spks --source=. --goarch=all --dsm-version=all
 
-pushspk: spk
+pushspk: spk ## Push and install synology package on ${SYNO_HOST} host
 	echo "Pushing SPK to root@${SYNO_HOST} (env var SYNO_HOST) ..."
 	scp tailscale.spk root@${SYNO_HOST}:
 	ssh root@${SYNO_HOST} /usr/syno/bin/synopkg install tailscale.spk
 
-publishdevimage:
+publishdevimage: ## Build and publish tailscale image to location specified by ${REPO}
 	@test -n "${REPO}" || (echo "REPO=... required; e.g. REPO=ghcr.io/${USER}/tailscale" && exit 1)
 	@test "${REPO}" != "tailscale/tailscale" || (echo "REPO=... must not be tailscale/tailscale" && exit 1)
 	@test "${REPO}" != "ghcr.io/tailscale/tailscale" || (echo "REPO=... must not be ghcr.io/tailscale/tailscale" && exit 1)
@@ -70,10 +67,18 @@ publishdevimage:
 	@test "${REPO}" != "ghcr.io/tailscale/k8s-operator" || (echo "REPO=... must not be ghcr.io/tailscale/k8s-operator" && exit 1)
 	TAGS=latest REPOS=${REPO} PUSH=true TARGET=client ./build_docker.sh
 
-publishdevoperator:
+publishdevoperator: ## Build and publish k8s-operator image to location specified by ${REPO}
 	@test -n "${REPO}" || (echo "REPO=... required; e.g. REPO=ghcr.io/${USER}/tailscale" && exit 1)
 	@test "${REPO}" != "tailscale/tailscale" || (echo "REPO=... must not be tailscale/tailscale" && exit 1)
 	@test "${REPO}" != "ghcr.io/tailscale/tailscale" || (echo "REPO=... must not be ghcr.io/tailscale/tailscale" && exit 1)
 	@test "${REPO}" != "tailscale/k8s-operator" || (echo "REPO=... must not be tailscale/k8s-operator" && exit 1)
 	@test "${REPO}" != "ghcr.io/tailscale/k8s-operator" || (echo "REPO=... must not be ghcr.io/tailscale/k8s-operator" && exit 1)
 	TAGS=latest REPOS=${REPO} PUSH=true TARGET=operator ./build_docker.sh
+
+help: ## Show this help
+	@echo "\nSpecify a command. The choices are:\n"
+	@grep -hE '^[0-9a-zA-Z_-]+:.*?## .*$$' ${MAKEFILE_LIST} | awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[0;36m%-20s\033[m %s\n", $$1, $$2}'
+	@echo ""
+.PHONY: help
+
+.DEFAULT_GOAL := help