Provide most stable public instances

[vitonsky]

Please, provide most stable public instances in readme.

I'm maintainer of NPM package https://github.com/translate-tools/core that unify translation API, provide translation primitives and translators implementations for most popular translation services.

Your service used by this package and sometimes fails tests. We have issue about this problem translate-tools/core#75

We need a stable public instances with high availability, to not mark translator that use your API are unstable.

Unstable translators are not visible for most users (but available to use) and may be removed any time in future releases with no notices.