chore(build): exclude nested README.md files from sdist packaging

This ensures that adding README.md files to subdirectories (as discussed
for new folders and integrations) won't result in them being included in
the published package.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 916075206

Author: GWeale

pyproject.toml

@@ -166,7 +166,7 @@ scripts.adk = "google.adk.cli:main"
 
 [tool.flit.sdist]
 include = [ 'src/**/*', 'README.md', 'pyproject.toml', 'LICENSE' ]
-exclude = [ 'src/**/*.sh' ]
+exclude = [ 'src/**/*.sh', 'src/**/README.md' ]
 
 [tool.flit.module]
 name = "google.adk"