update link text for fluentd official

Hi, @kiyoto 
Is it a quick and better solution to add link for fluentd official web page?

* original:
https://github.com/fluent/fluent-plugin-rewrite-tag-filter/blob/502f80f/README.md

* current: dropping out travis ci banner
https://github.com/fluent/fluent-plugin-rewrite-tag-filter/blob/6c0384d/README.md

* this PR
https://github.com/fluent/fluent-plugin-rewrite-tag-filter/blob/2b5fc30/README.md