Update README notes about repository structure
Update README notes about repository structure

Problem:

The notes have not been updated since we moved the demo app
to the `spec/example_app` directory.

The notes in the README are misleading.

Solution:

Update the repository structure section in the README.

thoughtbot/administrate,TheHumanEffort/administrate,astarix/administrate,nando/administrate,thoughtbot/administrate,vgsantoniazzi/administrate,micapam/administrate,BenMorganIO/administrate,tmecklem/administrate,kpheasey/administrate,acrogenesis-lab/administrate,kpheasey/administrate,BenMorganIO/administrate,acrogenesis-lab/administrate,flama/administrate,acrogenesis-lab/administrate,flama/administrate,BenMorganIO/administrate,thoughtbot/administrate,nburkley/administrate,kpheasey/administrate,vgsantoniazzi/administrate,micapam/administrate,nburkley/administrate,joshua5201/administrate,thoughtbot/administrate,BenMorganIO/administrate,nando/administrate,nburkley/administrate,TheHumanEffort/administrate,TheHumanEffort/administrate,tmecklem/administrate,astarix/administrate,nando/administrate,acrogenesis-lab/administrate,tmecklem/administrate,joshua5201/administrate,dminchev/administrate,kpheasey/administrate,dminchev/administrate,flama/administrate,TheHumanEffort/administrate,astarix/administrate,tmecklem/administrate,joshua5201/administrate,vgsantoniazzi/administrate,dminchev/administrate,joshua5201/administrate,micapam/administrate,micapam/administrate,dminchev/administrate,astarix/administrate,nburkley/administrate,vgsantoniazzi/administrate,flama/administrate,nando/administrate