Add help command to Makefile

[0xd669]

After reading this article, I guessed help command would be helpful. I just added some comments for each command and add help command. make help would print out the following result:

$ make help
Usage: 

 init           install requirements
 check          check if everything's okay
 format         format files
 test           run tests
 coverage       run tests with coverage
 htmlcov        run tests with coverage and create coverage report HTML files
 requirements   update requirements.txt
 help           prints this help message

[0xd669]

Oops, I guess tests for Makefile has broken. I shall fix it.

[winterjung]

Thank you for contribution! Please make tests pass

[winterjung]

LGTM