[DISCUSS] Api Since Version Annotation

[Jingsong Li]

Hi everyone,

I'm a bit curious why we don't use the api since annotation.

I found this annotation very useful when I am using other
dependencies, it tells me very directly the version of the API and
which API I used would break the compatibility of which version.

For example, String in JDK.

```
/*
 * ...
 * @since   JDK1.0
 */
public final class String {
    /*
     * @since 1.4
    */
    public CharSequence subSequence(...) {...}
}

```

Do you think this is helpful?

Best,
Jingsong