create Makefile for faster iteration

* makefile can automatically build the package, then navigate to the
  cpython repository to install the package and/or rebuild documentation

CONTRIBUTING.rst

@@ -13,3 +13,12 @@
 - bump version (YYYY.MM.dev) in setup.py and python_docs_theme/theme.conf
 - Commit this last bump.
 - push and push the tag (``git push && git push --tags``)
+
+# Makefile Usage
+
+This project includes a simple Makefile for syncing changes to the theme with
+the main cpython repository. Run ``make help`` for details on available rules.
+
+There is one configurable variable, ``CPYTHON_ROOT``, which should be the path
+to the cpython repository on your machine. By default, it points to
+``../cpython``.

Makefile

@@ -0,0 +1,21 @@
+CPYTHON_ROOT               = ../cpython
+PACKAGE_ABS_PATH = $(shell pwd)/$(shell find dist/python-docs-theme-*.tar.gz)
+
+
+.PHONY: all
+all: install
+	cd $(CPYTHON_ROOT)/Doc && \
+		make html
+
+
+.PHONY: install
+install:
+	python3 -m build
+	cd $(CPYTHON_ROOT)/Doc && \
+		./venv/bin/pip install $(PACKAGE_ABS_PATH)
+
+.PHONY: help
+help:
+	@echo "all: run the \`\`\install\`\\ rule, and also rebuild the cpython docs"
+	@echo "install: build the package, and install it in the virtual environment"
+	@echo "         at $(CPYTHON_ROOT)/Doc/venv"