add build:feature-config to readme instructions

[rrennick]

#1500 added npm build:feature-config to the build process. This PR adds that to the npm run build command.

Testing instructions

1. Delete includes/feature-config.php if it exists.
2. Run npm run build
3. Load the dashboard of the site without error.

[justinshreve]

Good catch on the missing step for npm run build and thanks for the fix! Should we instead update the command in package.json? so you can still just run npm run build and it will run both pieces?

[rrennick]

Should we instead update the command in package.json? so you can still just run npm run build and it will run both pieces?

Sounds like a better fix.

[rrennick]

@justinshreve Thanks. I updated the PR and testing instructions.

[justinshreve]

Looks good to me and is building correctly, and no fatal around the missing file. Thanks again!