Add doc: explain which files will be deleted when the maxFiles parameter is set

This will make it less confusing for developers.

Author: polunzh

README.md

@@ -26,7 +26,7 @@ The DailyRotateFile transport can rotate files by minute, hour, day, month, year
 * **dirname:** The directory name to save log files to. (default: '.')
 * **stream:** Write directly to a custom stream and bypass the rotation capabilities. (default: null)
 * **maxSize:** Maximum size of the file after which it will rotate. This can be a number of bytes, or units of kb, mb, and gb. If using the units, add 'k', 'm', or 'g' as the suffix. The units need to directly follow the number. (default: null)
-* **maxFiles:** Maximum number of logs to keep. If not set, no logs will be removed. This can be a number of files or number of days. If using days, add 'd' as the suffix. (default: null)
+* **maxFiles:** Maximum number of logs to keep. If not set, no logs will be removed. This can be a number of files or number of days. If using days, add 'd' as the suffix. It uses auditFile to keep track of the log files in a json format. It won't delete any file not contained in it. It can be a number of files or number of days (default: null)
 * **options:** An object resembling https://nodejs.org/api/fs.html#fs_fs_createwritestream_path_options indicating additional options that should be passed to the file stream. (default: `{ flags: 'a' }`)
 * **auditFile**: A string representing the name of the audit file. This can be used to override the default filename which is generated by computing a hash of the options object. (default: '.<optionsHash>.json')
 * **utc**: Use UTC time for date in filename. (default: false)