add documentation to makefile

Makefile

@@ -1,12 +1,21 @@
-.PHONY: docs
+.PHONY: help ## Print this help
+help:
+	@grep -E '^\.PHONY: [a-zA-Z_-]+ .*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = "(: |##)"}; {printf "\033[36m%-30s\033[0m %s\n", $$2, $$3}'
 
+.PHONY: run-tests ## Run unit tests
 run-tests:
 	pipenv run pytest tests
+
+.PHONY: init ## Initialize pipenv for development
 init:
 	python setup.py install
 	pipenv install --dev
+
+.PHONY: docs ## Generate documentation
 docs:
 	cd docs && make html
+
+.PHONY: ## Generate man documentation
 man:
 	cd docs && make man
 	mv docs/_build/man/pipenv.1 pipenv/pipenv.1