Build: Remove --production flag to fix grunt prepare

I don't know how long this has been broken for, but, `--production`
is incompatible with grunt-check-modules as used in the
download.jqueryui.com package for the `grunt prepare` step.

Test Plan
* `apt-get install -y libxml2-utils xsltproc imagemagick`
* Create `config.json`
* Run `npx grunt build`

```
Running "build-download" task
Initializing download module, might take a while...
>> Running "check-modules" task
>> Warning: Command failed: npm ls
>> npm ERR! code ELSPROBLEMS
>> npm ERR! missing: [email protected], required by [email protected]
```

Gruntfile.js

@@ -64,7 +64,9 @@ grunt.registerTask( "build-download", function() {
 
 	// At this point, the download builder repo is available, so let's initialize it
 	grunt.log.writeln( "Initializing download module, might take a while..." );
-	exec( "npm install --production", {
+	// TODO: Prefer --omit=dev but can't because `grunt prepare` runs grunt-check-modules,
+	// which fails if a dev dependency is missing.
+	exec( "npm install", {
 		cwd: "node_modules/download.jqueryui.com"
 	}, function( error, stdout, stderr ) {
 		if ( error ) {

README.md

@@ -3,3 +3,10 @@
 ## Building and Deploying
 
 To build and deploy your changes for previewing in a [`jquery-wp-content`](https://github.com/jquery/jquery-wp-content) instance, follow the [workflow instructions](http://contribute.jquery.org/web-sites/#workflow) from our documentation on [contributing to jQuery Foundation web sites](http://contribute.jquery.org/web-sites/).
+
+Prerequisites:
+* `apt-get install -y libxml2-utils xsltproc imagemagick`
+* Create `config.json`
+
+Build the dist directory locally:
+* `npx grunt build`