chore: add link to ref docs in README (#2384)

Author: bshaffer

README.md

@@ -2,6 +2,10 @@
 
 # Google APIs Client Library for PHP #
 
+**NOTE**: please check to see if the package you'd like to install is available in our
+list of [Google cloud packages](https://cloud.google.com/php/docs/reference) first, as
+these are the recommended libraries.
+
 <dl>
   <dt>Reference Docs</dt><dd><a href="https://googleapis.github.io/google-api-php-client/main/">https://googleapis.github.io/google-api-php-client/main/</a></dd>
   <dt>License</dt><dd>Apache 2.0</dd>