Update getting started section to include npm install

[fabiankaegy]

If you currently follow the getting started section step by step the first thing you are getting is an error that he node modules are not installed.

This of course is the case, but I believe it might be more frictionless for first time contributors when every step is included in the list.

Changes proposed in this Pull Request

• Added step 4 to the getting started list

[jsnajdr]

When I run npm start on a repo without installed node_modules, the prestart script in package.json does the install for me:

> wp-calypso@0.17.0 prestart /Users/jsnajdr/src/wp-calypso
> npx check-node-version --package && npm run -s install-if-deps-outdated && node bin/welcome.js

The install-if-deps-outdated script does that. Also, the right install command is npm ci, not npm install. npm ci fully respects the lockfile and is appropriate for situations like this.

What errors are you getting when running npm start without installing first?

[fabiankaegy]

Okay found my error. Had version 12.5.0 of node running

node: 12.5.0
Error: Wanted node version ^10.16.3 (>=10.16.3 <11.0.0)
To install node, run `nvm install ^10.16.3` or see https://nodejs.org/

[jsnajdr]

@fabiankaegy We are working on the Node 12 upgrade in #37031. You can help testing or make some other contribution, if you're interested.

Also, maybe we can improve the experience by better error messages, so that it's more obvious what is the issue?