Add github.com/mitchellh/mapstructure.

In mitchellh/mapstructure#24, the author said he wouldn't mind it here. It's a high quality Go library that serves a specific need.

README.md

@@ -451,6 +451,7 @@ Join us on IRC at **#awesome-go** on freenode [web access](http://webchat.freeno
 * [cbor](https://github.com/2tvenom/cbor) - Golang library for working with cbor binary format
 * [gogoprotobuf](https://github.com/gogo/protobuf) - Protocol Buffers for Go with Gadgets
 * [goprotobuf](https://github.com/golang/protobuf) - Go support, in the form of a library and protocol compiler plugin, for Google's protocol buffers.
+* [mapstructure](https://github.com/mitchellh/mapstructure) - Go library for decoding generic map values into native Go structures.
 * [php_session_decoder](https://github.com/yvasiyarov/php_session_decoder) - GoLang library for working with PHP session format and PHP Serialize/Unserialize functions