Clearer instructions for using the Makefile

[tompollard]

It would be helpful to provide instructions for using the Makefile, perhaps in the Readme.md. Also consider including a help rule in the Makefile to print usage instructions when someone tries to run make with no arguments. e.g. add the following chunk above all:

help:
	@echo '----------------------------------------------------------------------------'
	@echo ' Makefile for de-identifying clinical data '
	@echo '                                                                            '
	@echo ' Usage: '
	@echo '                                                                            '
	@echo ' Deidentify a dataset in the X directory:  '
	@echo '                                                                            '
	@echo '             make all                         '
	@echo '                                                                            '
	@echo ' Run the deidentification script against a test dataset:  '
	@echo '                                                                            '
	@echo '             make test                         '
	@echo '----------------------------------------------------------------------------'
	@echo '                                                                            '