docs(postprocessor): nudge users towards stable clients (#8513)

Explicitly call out in our READMEs what a stable client is and recommend using it when available.

In a future PR I will cleanup readmes in batch.

Internal bug: 288889032

internal/postprocessor/_README.md.txt

@@ -19,6 +19,17 @@ However, a `v1+` module may have breaking changes in two scenarios:
 * Packages with `alpha` or `beta` in the import path
 * The GoDoc has an explicit stability disclaimer (for example, for an experimental feature).
 
+### Which package to use?
+
+Generated client library surfaces can be found in packages who's import path
+ends in `.../apivXXX`. The `XXX` could be something like `1` or `2` in the case
+of a stable service backend or may be like `1beta2` or `2beta` in the case of a
+more experimental service backend. Because of this fact, a given module can have
+multiple clients for different service backends. In these cases it is generally
+recommend to use clients with stable service backends, with import suffixes like
+`apiv1`, unless you need to use features that are only present in a beta backend
+or there is not yet a stable backend available.
+
 ## Google Cloud Samples
 
 To browse ready to use code samples check [Google Cloud Samples](https://cloud.google.com/docs/samples?l=go).