Update README.md to recommend a newer version of Node

[ispivey]

The README currently says you need to be on node version 9. However, that doesn't work with the new stack:

➜  workers-docs git:(template-content) npm run start

> cloudflare-workers-docs@1.0.0 start /Users/ispivey/cf-repos/workers-docs
> npm run develop


> cloudflare-workers-docs@1.0.0 develop /Users/ispivey/cf-repos/workers-docs
> gatsby develop -p 1313


 ERROR

Gatsby requires Node.js 10.13.0 or higher (you have v9.11.2).
Upgrade Node to the latest stable release: https://gatsby.dev/upgrading-node-js

We should update the README to reflect the desired version, e.g. version 10.13.0 or higher, or remove the version.