Infra: Add 'make help' (#2858)

Makefile

@@ -6,34 +6,42 @@ JOBS=8
 OUTPUT_DIR=build
 RENDER_COMMAND=$(VENVDIR)/bin/python3 build.py -j $(JOBS) -o $(OUTPUT_DIR)
 
+## render         to render PEPs to "pep-NNNN.html" files
 .PHONY: render
 render: venv
 	$(RENDER_COMMAND)
 
+## pages          to render PEPs to "index.html" files within "pep-NNNN" directories
 .PHONY: pages
 pages: venv rss
 	$(RENDER_COMMAND) --build-dirs
 
+## fail-warning   to render PEPs to "pep-NNNN.html" files and fail the Sphinx build on any warning
 .PHONY: fail-warning
 fail-warning: venv
 	$(RENDER_COMMAND) --fail-on-warning
 
+## check-links    to check validity of links within PEP sources
 .PHONY: check-links
 check-links: venv
 	$(RENDER_COMMAND) --check-links
 
+## rss            to generate the peps.rss file
 .PHONY: rss
 rss: venv
 	$(VENVDIR)/bin/python3 generate_rss.py
 
+## clean          to remove the venv and build files
 .PHONY: clean
 clean: clean-venv
 	-rm -rf build topic
 
+## clean-venv     to remove the venv
 .PHONY: clean-venv
 clean-venv:
 	rm -rf $(VENVDIR)
 
+## venv           to create a venv with necessary tools
 .PHONY: venv
 venv:
 	@if [ -d $(VENVDIR) ] ; then \
@@ -46,16 +54,24 @@ venv:
 		echo "The venv has been created in the $(VENVDIR) directory"; \
 	fi
 
+## lint           to lint all the files
 .PHONY: lint
 lint: venv
 	$(VENVDIR)/bin/python3 -m pre_commit --version > /dev/null || $(VENVDIR)/bin/python3 -m pip install pre-commit
 	$(VENVDIR)/bin/python3 -m pre_commit run --all-files
 
+## test           to test the Sphinx extensions for PEPs
 .PHONY: test
 test: venv
 	$(VENVDIR)/bin/python3 -bb -X dev -W error -m pytest
 
+## spellcheck     to check spelling
 .PHONY: spellcheck
 spellcheck: venv
 	$(VENVDIR)/bin/python3 -m pre_commit --version > /dev/null || $(VENVDIR)/bin/python3 -m pip install pre-commit
 	$(VENVDIR)/bin/python3 -m pre_commit run --all-files --hook-stage manual codespell
+
+.PHONY: help
+help : Makefile
+	@echo "Please use \`make <target>' where <target> is one of"
+	@sed -n 's/^##//p' $<