Add developer docs.

[hildjj]

Fixes #466.

[hildjj]

@frostburn please review.

[frostburn]

Looks good and got the project up and running on my end. 👍

Additional remarks

1. It would be nice to link to CONTRIBUTING.md in README.md.
2. After the setup described in this PR I got untracked files docs/package-lock.json and web-test/package-lock.json. These should be either included in the repository or added to .gitignore.
3. It might've been npm run lint but docs/js/test-bundle.min.js ends up modified on my end according to git.

[hildjj]

1. willfix
2. willfix, was playing with workspaces, but decided I didn't like them.
3. I might not have checked in my last test fixes minified. willfix.