Add screenshot of API documentation in README

Author: luismtorresv

.docs/screenshots/ArsLonga-screenshot-docs_api.png

@@ -103,6 +103,8 @@ automatically generated by [Scramble][scramble].
 
 [scramble]: <https://scramble.dedoc.co/> "Scramble – Laravel OpenAPI (Swagger) Documentation Generator"
 
+![Ars Longa API documentation.](.docs/screenshots/ArsLonga-screenshot-docs_api.png)
+
 ## Authors
 
 1. Jerónimo Acosta