fix: Example allows manifest from self

[MichalBryxi]

• Given example is quite handy starting copy&paste.
• Unfortunatelly for Chrome with modern Ember apps it gives:

assessment:1 Refused to load manifest from 'http://localhost:4200/manifest.webmanifest' because it violates the following Content Security Policy directive: "default-src 'none'". Note that 'manifest-src' was not explicitly set, so 'default-src' is used as a fallback.

• This patch makes the README as a nicer starting point.

[jelhan]

I'm wondering if we should update the default configuration as well:

ember-cli-content-security-policy/lib/utils.js

Lines 73 to 87 in b6c3e09

	let config = {
	delivery: ['header'],
	enabled: true,
	failTests: true,
	policy: {
	'default-src': [CSP_NONE],
	'script-src': [CSP_SELF],
	'font-src': [CSP_SELF],
	'connect-src': [CSP_SELF],
	'img-src': [CSP_SELF],
	'style-src': [CSP_SELF],
	'media-src': [CSP_SELF],
	},
	reportOnly: true,
	};

and

ember-cli-content-security-policy/README.md

Lines 94 to 116 in b6c3e09

	If you omit some or all of the keys, the default configuration will be used, which is:
	```js
	// config/content-security-policy.js
	module.exports = function (environment) {
	return {
	delivery: ['header'],
	enabled: true,
	failTests: true,
	policy: {
	'default-src': ["'none'"],
	'script-src': ["'self'"],
	'font-src': ["'self'"],
	'connect-src': ["'self'"],
	'img-src': ["'self'"],
	'style-src': ["'self'"],
	'media-src': ["'self'"],
	},
	reportOnly: true,
	};
	};
	```

[MichalBryxi]

Good point @jelhan. Added.

[MichalBryxi]

Not sure why the CI gods are angry ...

[jelhan]

Not sure why the CI gods are angry ...

It seems to fail due to Prettier formatting:

$ eslint . --cache

/home/runner/work/ember-cli-content-security-policy/ember-cli-content-security-policy/lib/utils.js
ERROR: "lint:js" exited with 1.
Error:   78:7  error  Replace `·'default-src':·` with `'default-src':`                prettier/prettier
Error:   79:7  error  Replace `·'script-src':···` with `'script-src':·`               prettier/prettier
Error:   80:7  error  Replace `·'font-src':·····` with `'font-src':·`                 prettier/prettier
Error:   81:1  error  Replace `·······'connect-src':··` with `······'connect-src':·`  prettier/prettier
Error:   82:7  error  Replace `·'img-src':······` with `'img-src':·`                  prettier/prettier
Error:   83:7  error  Replace `·'style-src':····` with `'style-src':·`                prettier/prettier
Error:   84:1  error  Delete `·`                                                      prettier/prettier
Error:   85:7  error  Replace `·'media-src':···` with `'media-src':`                  prettier/prettier

✖ 8 problems (8 errors, 0 warnings)
  8 errors and 0 warnings potentially fixable with the `--fix` option.

[MichalBryxi]

Even change in Readme would mean running CI, which is borked now. Just going to close this for now.