Added contributing docs and Makefile for doing checks

Makefile

@@ -0,0 +1,33 @@
+default: help
+
+.PHONY: help
+help:   ## show this help
+	@echo 'usage: make [target] ...'
+	@echo ''
+	@echo 'targets:'
+	@egrep '^(.+)\:\ .*##\ (.+)' ${MAKEFILE_LIST} | sed 's/:.*##/#/' | column -t -c 2 -s '#'
+
+.PHONY: check
+check: check-format test lint  ## run all code checks
+
+.PHONY: lint
+lint:   ## lint the code with golangci-lint
+	golangci-lint run
+
+.PHONY: test
+test:   ## test the code with go test
+	go test -v ./...
+
+.PHONY: format
+format: ## format the code with goimports and tidy the go modules
+	go mod tidy -v
+	goimports -local git.maze.io/go/dpi -w *.go */*.go
+
+.PHONY: check-format
+check-format: ## check if the the code formatting and modules
+	go mod tidy -diff
+	goimports -local git.maze.io/go/dpi -l -w *.go */*.go
+
+.PHONY: tools
+tools:  ## fetch and install all required tools
+	go get -v -u golang.org/x/tools/cmd/goimports@latest