Docs: Add JSON output format to inital introduction sentence (#580)

Co-authored-by: L. Wagner <[email protected]>

Author: LitschiW

README.md

@@ -1,6 +1,6 @@
 # OpenAPI-diff 
 
-Compare two OpenAPI specifications (3.x) and render the difference to HTML plaintext, or Markdown files.
+Compare two OpenAPI specifications (3.x) and render the difference to HTML plaintext, Markdown files, or JSON files.
 
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