docs(how-to): finish

[nokome]

Current draft in how-to.md at top level.

(Just creating a list of issues to be completed for https://github.com/stencila/dockter/milestone/2)

[apawlik]

1. Rename the folder to examples
2. Rename how-to to an example case study or similar
3. The file should describe:

• Writing a Dockerfile from scratch manually
• Discuss what is not captured / easily missed
• Discuss the tools which support building Dockerfiles
• Discuss what they may not capture (like system dependencies)
• Discuss Dockter