Add explanations

CONTRIBUTING.md

@@ -69,6 +69,14 @@ You can run:
   Python version
 - `tox -e lint` to run lint checks on all code
 
+We try to keep the amount of _public symbols_ in our code minimal. A public symbol is any Python identifier that does not start with an underscore.
+Every public symbol is something that has to be kept in order to maintain backwards compatibility, so we try to have as few as possible.
+
+To check if your PR is adding public symbols, run `tox -e public-symbols-check`. This will always fail if public symbols are being added. The idea
+behind this is that every PR that adds public symbols fails in CI, forcing reviewers to check the symbols to make sure they are strictly necessary.
+If after checking them, it is considered that they are indeed necessary, the PR will be labeled with `Skip Public API check` so that this check is not
+run.
+
 See
 [`tox.ini`](https://github.com/open-telemetry/opentelemetry-python/blob/main/tox.ini)
 for more detail on available tox commands.