update readme and mention gopkg.in in godoc

lumberjack.go

@@ -1,5 +1,13 @@
 // Package lumberjack provides a rolling logger.
 //
+// Note that this is v2.0 of lumberjack, and should be imported using gopkg.in
+// thusly:
+//
+//   import "gopkg.in/natefinch/lumberjack.v2"
+//
+// The package name remains simply lumberjack, and the code resides at
+// https://github.com/natefinch/lumberjack under the v2.0 branch.
+//
 // Lumberjack is intended to be one part of a logging infrastructure.
 // It is not an all-in-one solution, but instead is a pluggable
 // component at the bottom of the logging stack that simply controls the files