Improve installation instructions

[x] user should run pip3 install -r requirements.txt before import electiondata as ea
[x] user needs a postgreSQL instance (e.g., on macOS with homebrew package manager, brew install postgresql and login information for a user on that instance (with proper permissions)
[x] user does not need to create a database in their postgreSQL instance -- program will do that.
[x] required packages may have dependencies (e.g., matplotlib requires freetype). User will need to use their package manager (e.g., might be homebrew on a macOS system).

ElectionDataAnalysis / electiondata