docs: Fix npmjs link rendering in README.md
docs: Fix npmjs link rendering in README.md

npmjs.com will only link back to GitHub if those links begin with a "/" character.  This fixes the way the README appears on npmjs.com, so that all links to other docs will link back to GitHub.

shaka-project/generic-webdriver-server,shaka-project/generic-webdriver-server,shaka-project/generic-webdriver-server,shaka-project/generic-webdriver-server,shaka-project/generic-webdriver-server