Fix broken links in README.md

[sjwaight]

Purpose

Sample code, zip file and key image were all previously broken in the markdown. This submission fixes those links.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

[sjwaight]

Flagging this with @TonyXing. There are a few PRs awaiting action.