Add pkg/errors. (#1113)

Package errors provides simple error handling primitives.

README.md

@@ -601,6 +601,7 @@ See [go-hardware](https://github.com/rakyll/go-hardware) for a comprehensive lis
 * [bitio](https://github.com/icza/bitio) - Highly optimized bit-level Reader and Writer for Go.
 * [browscap_go](https://github.com/digitalcrab/browscap_go) - GoLang Library for [Browser Capabilities Project](http://browscap.org/).
 * [datacounter](https://github.com/miolini/datacounter) - Go counters for readers/writer/http.ResponseWriter.
+* [errors](https://github.com/pkg/errors) - A package that provides simple error handling primitives.
 * [go-chat-bot](https://github.com/go-chat-bot/bot) - IRC, Slack & Telegram bot written in Go.
 * [go-commons-pool](https://github.com/jolestar/go-commons-pool) - A generic object pool for Golang.
 * [go-multierror](https://github.com/hashicorp/go-multierror) - A Go (golang) package for representing a list of errors as a single error.