docs(readme): improve api reference (#139)

README.md

@@ -12,7 +12,7 @@ The Lithic Java SDK is similar to the Lithic Kotlin SDK but with minor differenc
 
 ## Documentation
 
-The API documentation can be found [here](https://docs.lithic.com).
+The REST API documentation can be found [on docs.lithic.com](https://docs.lithic.com). The full API of this library can be found in [api.md](https://www.github.com/lithic-com/lithic-java/blob/main/api.md).
 
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