📚 [README] Clarify how development dependencies are installed

README.rst

@@ -92,6 +92,14 @@ For more fine-grained control, additional utilities are available under the ``se
 
 Note that ``distribution_format`` is a `keyword-only parameter`_.
 
+.. note::
+
+   Use ``session.install`` to install specific packages from your development
+   dependencies, and ``session.install(".")`` to install your own package. Use
+   ``session.run_always("poetry", "install", external=True)`` to install your
+   package with *all* development dependencies. Please read the next section for
+   the tradeoffs of each method.
+
 
 Why?
 ----
@@ -120,7 +128,7 @@ and installing your package and its dependencies using ``poetry install``:
    @nox.session
    def tests(session: Session) -> None:
        """Run the test suite."""
-       session.run("poetry", "install", external=True)
+       session.run_always("poetry", "install", external=True)
        session.run("pytest")
 
 Unfortunately, this approach comes with its own set of problems: