Update README.md

[MoChilia]

#57 changed the default value for azcliversion from latest to the version of Azure CLI installed on agent. If there is no version of Azure CLI found on the agent, the action falls back the version to latest.

cli/src/main.ts

Lines 32 to 44 in bb23033

	if (azcliversion == AZ_CLI_VERSION_DEFAULT_VALUE) {
	try {
	const { stdout, stderr } = await cpExec('az version');
	if (!stderr) {
	azcliversion = JSON.parse(stdout)["azure-cli"]
	} else {
	throw stderr
	}
	} catch (err) {
	console.log('Failed to fetch az cli version from agent. Reverting back to latest.')
	azcliversion = 'latest'
	}
	}

This pr will update the description about default version used in azure/cli and remove the 'Configure Azure credentials as GitHub Secret' section, since it is not related to azure/cli action, but should be found in https://github.com/Azure/login/blob/master/README.md. And 'Azure CLI Action metadata file' section will be removed, since it is unnecessary to appear in README.md.