Render interface broken contract in #543

[DrSatyr]

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.