update docs w/ backup format info

README.md

@@ -87,6 +87,14 @@ the current file is closed, renamed, and a new log file created with the
 original name. Thus, the filename you give Logger is always the "current" log
 file.
 
+Backups use the log file name given to Logger, in the form `name-timestamp.ext`
+where name is the filename without the extension, timestamp is the time at which
+the log was rotated formatted with the time.Time format of
+`2006-01-02T15-04-05.000` and the extension is the original extension.  For
+example, if your Logger.Filename is `/var/log/foo/server.log`, a backup created
+at 6:30pm on Nov 11 2016 would use the filename
+`/var/log/foo/server-2016-11-04T18-30-00.000.log`
+
 ### Cleaning Up Old Log Files
 Whenever a new logfile gets created, old log files may be deleted.  The most
 recent files according to the encoded timestamp will be retained, up to a

lumberjack.go

@@ -55,6 +55,14 @@ var _ io.WriteCloser = (*Logger)(nil)
 // original name. Thus, the filename you give Logger is always the "current" log
 // file.
 //
+// Backups use the log file name given to Logger, in the form
+// `name-timestamp.ext` where name is the filename without the extension,
+// timestamp is the time at which the log was rotated formatted with the
+// time.Time format of `2006-01-02T15-04-05.000` and the extension is the
+// original extension.  For example, if your Logger.Filename is
+// `/var/log/foo/server.log`, a backup created at 6:30pm on Nov 11 2016 would
+// use the filename `/var/log/foo/server-2016-11-04T18-30-00.000.log`
+//
 // Cleaning Up Old Log Files
 //
 // Whenever a new logfile gets created, old log files may be deleted.  The most