Document how to compile Bootstrap5 subtheme

I've only just started looking at the theme, but I think those instructions are if you're going to help develop the main theme.

There's a README in your subtheme that tells you how to compile. FYI it doesn't need a package.json/npm install

Subtheme doesn't have npm tools.
Please look after your own sass binary.
The instructions on how to install sass can be found here: https://sass-lang.com/

Thanks @maseyuk, you're correct. npm install is not needed to recompile a sub-theme.

@VladimirAus: I have Sass installed, the problem was that the initial npm install is not needed for a sub-theme. I'll create a MR for the README file.

I added sub-theme recompile instructions in the README file, which is probably where the users expect to find them.

I also made minor updates, to align more with the README template → .

As opposed to the v3 theme which mentions using npm the v4 readme now just mentions the basic SASS command. This is probably good enough, the main theme has all the creation stuff in it (which someone would have already done). I'm tempted to flag this as outdated at the moment.

### CSS compilation.

Prerequisites: install [sass](https://sass-lang.com/install).

To compile, run from subtheme directory: `sass scss/style.scss css/style.css && sass scss/ck5style.scss css/ck5style.css`

If anyone would find it useful, here's the project config I'm using for https://marketplace.visualstudio.com/items?itemName=glenn2223.live-sass in VSCode for this theme. :)

"settings": {
		"liveSassCompile.settings.compileOnWatch": false,
		"liveSassCompile.settings.watchOnLaunch": true,
		"liveSassCompile.settings.useNewCompiler": true,
		"liveSassCompile.settings.showOutputWindowOn": "Warning",
		"liveSassCompile.settings.formats": [
			{
				"format": "expanded",
				"extensionName": ".css",
				"savePath": null,
				"savePathReplacementPairs": {
					"/scss/": "/css"
				}
			},
			{
				"format": "compressed",
				"extensionName": "-min.css",
				"savePath": null,
				"savePathReplacementPairs": {
					"/scss/": "/css"
				},
				"generateMap": false
			}
		]
	}