[pkg] Add README documenting the purpose of the folder (open-telemetr…

…y#12650)

pkg/README.md

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+# `pkg` folder
+
+The Go modules in this folder can help in building new components (receivers, processors, exporters, extensions...) and are meant for **external** consumption.
+Carefully review the public API to ensure that we only expose what is deemed necessary, so that we can easily change the code without affecting external users.
+
+Breaking changes can happen on any module on this folder under the [opentelemetry-collector contribution guidelines](https://github.com/open-telemetry/opentelemetry-collector/blob/main/CONTRIBUTING.md#breaking-changes).
+
+If you are introducing a new module that will only be used by components in the opentelemetry-collector-contrib repository, consider placing it on the `internal` folder instead.
+This will reduce our public API footprint and maintenance burden.