Update Quickstart documentation

[janetriley]

Update Quickstart documentation as we discover gaps at PyCon sprints.

[janetriley]

• Add mention of Mongo 3.x, Python 2.7
• add docker build instructions: docker build -t "baleen_app_1" -f Dockerfile-app
• The test libraries are commented out in requirements.txt (and should stay that way so it isn't committed back accidentally. ) The make test step fails. Add a line between pip install -r requirements.txt for pip install nose==1.3.7 mongomock==3.2.1 coverage==4.0.3

[echolabstech]

Install and start mongoDB: https://docs.mongodb.com/manual/installation/

[bbengfort]

@dongbohu will take this issue.

[will2041]

I started making some notes on this as I went through the guide as well. Here's my dump, just for reference: readme_diff.txt