Add section clarifying pip version support (#11556)

Co-authored-by: Paul Moore <[email protected]>

Author: hollabaq86

docs/html/development/release-process.rst

@@ -65,6 +65,13 @@ their merits.
   ``pip._internal.utils.deprecation.deprecated``. The function is not a part of
   pip's public API.
 
+Supported Versions
+==================
+
+The latest version of the pip is the only supported version, previous
+versions should be considered unsupported. Users are encouraged to make
+regular updates to their version of pip in order to remain supported.
+
 .. _`Python 2 Support`:
 
 Python 2 Support