feat(cdn): add docs for refreshing with specified env vars

[yaonyan]

CDN 缓存刷新插件需要指定环境变量才能生效，

serverless-demo/Nodejs12.16-COSCdnRefresh/src/common/utils.js

Lines 10 to 44 in 55aade0

	function getParams(event) {
	const {
	TENCENTCLOUD_SECRETID: secretId,
	TENCENTCLOUD_SECRETKEY: secretKey,
	TENCENTCLOUD_SESSIONTOKEN: token,
	Records: records,
	ObjectInfo: objectInfo,
	...args
	} = {
	...process.env,
	...event,
	};
	let objects = [];
	let triggerType = '';
	const missingParams = [
	{ key: 'TENCENTCLOUD_SECRETID', value: secretId },
	{ key: 'TENCENTCLOUD_SECRETKEY', value: secretKey },
	// { key: 'TENCENTCLOUD_SESSIONTOKEN', value: token },
	]
	.filter(item => !item.value)
	.map(item => item.key);
	if (missingParams.length) {
	throw new Error(`params parsed error, missing params: ${missingParams.join(', ')}`);
	}
	const cdnHosts = [];
	for (const key in args) {
	if (/^cdnHosts/.test(key)) {
	cdnHosts.push(args[key]);
	}
	}

此 PR 声明这个隐式依赖