Add breaking changes note to README and issue template.

Author: tillahoffmann

.github/ISSUE_TEMPLATE/bug-or-unexpected-behavior.md

@@ -7,6 +7,8 @@ assignees: ''
 
 ---
 
+⚠️ testcontainers-python version 4.0.0 introduces breaking changes. Please review the note in the documentation (https://testcontainers-python.readthedocs.io/) before filing a bug.
+
 **Describe the bug**
 
 A clear and concise description of what the bug is. What did you expect to happen? What happened instead?
@@ -24,7 +26,7 @@ raise RuntimeError("something went wrong")
 Provide a summary of your runtime environment. Which operating system, python version, and docker version are you using? What is the version of `testcontainers-python` you are using? You can run the following commands to get the relevant information.
 
 ```bash
-# Get the operating system information (on a unix os).
+# Get the operating system information (e.g., on a unix os).
 $ uname -a
 # Get the python version.
 $ python --version

README.rst

@@ -8,7 +8,16 @@ testcontainers-python
 .. image:: https://readthedocs.org/projects/testcontainers-python/badge/?version=latest
    :target: http://testcontainers-python.readthedocs.io/en/latest/?badge=latest
 
-testcontainers-python facilitates the use of Docker containers for functional and integration testing. The collection of packages currently supports the following features.
+testcontainers-python facilitates the use of Docker containers for functional and integration testing.
+
+.. note::
+
+    testcontainers-python version 4.0.0 introduces several breaking changes. To use an older version, add :code:`testcontainers-python<4` :code:`install_requires` in your :code:`setup.py` if you are building a package or your :code:`requirements.txt` for pinned dependencies.
+
+    - Container arguments, such as user name, database name, and ports have been unified across containers in `testcontainers/testcontainers-python#296 <https://github.com/testcontainers/testcontainers-python/pull/296>`__. You may have to update your code if you modify the default values. The unified arguments are :code:`username`, :code:`password`, :code:`dbname`, and :code:`port`. If a container exposes more than one port, the port name is :code:`[specific_name]_port`.
+    - Class-level configuration variables have been removed in `testcontainers/testcontainers-python#296 <https://github.com/testcontainers/testcontainers-python/pull/296>`__, and configuration variables should be specified using environment variables (as before) or specified in the constructor.
+
+The collection of packages currently supports the following features.
 
 .. toctree::