give a clue to user on first install

[ianozsvald]

Add a docstring to the package and add %ipython_memory_usage_start to give the user a clue.

Update init.py with a """docstring that tells the user how to get started""" so that when they do:

import ipython_memory_usage 
ipython_memory_usage?

they get a couple of lines telling them to start with %ipython_memory_usage_start or %ipython_memory_usage_stop to show memory changes on each typed command