Render interface broken contract in #543

Since #543 Render contract was changed, it introduces backward incompatibility. 
```
String render(ChangedOpenApi diff);  -> void render(ChangedOpenApi diff, OutputStreamWriter outputStreamWriter);
```
We have to keep an old contract alongside with new one or add a migration guide to the 2.1.0 release notes. 
Also, these changes are not reflected in Readme.
