added a note about experimental features

Author: fabpot

contributing/code/bc.rst

@@ -32,6 +32,14 @@ The second section, "Working on Symfony Code", is targeted at Symfony
 contributors. This section lists detailed rules that every contributor needs to
 follow to ensure smooth upgrades for our users.
 
+.. warning::
+
+    Code marked with the ``@experimental`` or the ``@internal`` tags is
+    excluded from our Backward Compatibility promise.
+
+    Also note that backward compatibility breaks are tolerated if they are
+    required to fix a security issue.
+
 Using Symfony Code
 ------------------
 

contributing/community/releases.rst

@@ -150,6 +150,10 @@ of Symfony to the next one.
 Whenever keeping backward compatibility is not possible, the feature, the
 enhancement or the bug fix will be scheduled for the next major version.
 
+However, features marked with the ``@experimental`` tag are **not** part of the
+backward compatibility promise and their APIs can change from one minor version
+to the next. The changelog must explain the changes and how to upgrade.
+
 Deprecations
 ------------