Use the atx syntax for the headings in README.md.

Using atx instead of Setext syntax permits 6 heading levels (#..######)
instead of just 2 (====== and ------).
Improve the "Manual Deployment" section in README.md.
Improve the "Setup" section in README.md.
Improve the section "Local testing in Docker" in README.md.
Rename & improve the section "Deployment to Amazon EC2" to "Deploying to virtual machines".
Merge pull request #15 from stairclimber/pull-request-1

Improve the README.md