docs(readme): add swift support

[hongbo-miao]

Just a small update 😃

Based on

codeql-action/src/languages.ts

Lines 1 to 11 in 7e73ded

	// All the languages supported by CodeQL
	export enum Language {
	csharp = "csharp",
	cpp = "cpp",
	go = "go",
	java = "java",
	javascript = "javascript",
	python = "python",
	ruby = "ruby",
	swift = "swift",
	}

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[hongbo-miao]

I am getting error during CodeQL GitHub Action at here

A fatal error occurred: There's no CodeQL extractor named 'swift' installed.

Seems not general available based on #1369 unless passing extra env. So I will close for now.

[angelapwen]

Thank you for the contribution @hongbo-miao! That's correct — we won't update the list of possible languages in the README and starter workflow until the language support is in GA without the extra experimental environment variable passed. Appreciate your looking out though ✨