Makefile usage help

Makefile

@@ -1,3 +1,11 @@
+help:
+	@echo "Skyfield data - Makefile targets (devs only)"
+	@echo
+	@echo " * download: Download all files if necessary, default options."
+	@echo "             (see \`python download.py --help\` for options)"
+	@echo " * clean: remove all skyfield data from data directory."
+	@echo " * package: build python source package."
+
 download:
 	python3 download.py
 

README.md

@@ -31,6 +31,10 @@ load = Loader(get_skyfield_data_path())
 planets = load('de421.bsp')
 ```
 
+## Developers
+
+We're providing a ``Makefile`` with basic targets to play around with the toolkit. use ``make help`` to get more details.
+
 ## Copyright
 
 ### Data files