Update README.md (#1427)

Adding API Documentation Link

Author: bobstrecansky

README.md

@@ -10,6 +10,8 @@ This is the **[monorepo](https://en.wikipedia.org/wiki/Monorepo)** for the **mai
 
 Please read the official documentation: https://opentelemetry.io/docs/instrumentation/php/
 
+API Documentation is available here: https://open-telemetry.github.io/opentelemetry-php/
+
 ## Packages and versions
 
 | Package              | Latest                                                                                                                                                                                                                                                                                                                                                  |