Igbot: DOCUMENTATION: Add text about Instabot's filtration.

[ohld]

We should add a doc file with text how our bot filter the users and medias to follow and like.
All the code here https://github.com/instagrambot/instabot/blob/master/instabot/bot/bot_filter.py should be converted to the understandable words. Also the bot = Bot() constructor arguments should be added in this doc file.

Any volunteers?

[penarubah]

want me to do this?

[ohld]

@penarubah Do this if you want.