Docs: make the welcome page more obvious

Docs: make the welcome page more obvious
change only README.md

related #39
When it corresponds to a release, the link should be to the given version. So instead of `https://github.com/elasticsearch/elasticsearch-analysis-kuromoji/tree/es-1.3/` it should be `https://github.com/elasticsearch/elasticsearch-analysis-kuromoji/tree/v2.3.0/`.

BTW, `v2.3.0` was an old fashion doc release. So the anchor is: `#japanese-kuromoji-analysis-for-elasticsearch`

So link should be: `[2.3.0](https://github.com/elasticsearch/elasticsearch-analysis-kuromoji/tree/v2.3.0/#japanese-kuromoji-analysis-for-elasticsearch)`