Update MANIFEST_TEMPLATE.in.jinja2

[eladkal]

During provider release I noticed several warnnings:

Generated regular package setup files for google
########## Prepare provider package for 'google' ##########
Changing directory to /opt/airflow/provider_packages
Building provider package: google in format both
Executing command: 'python3 setup.py build --build-temp /tmp/tmplaa_85oz egg_info --tag-build sdist bdist_wheel --bdist-dir /tmp/tmpyjwnfmkg'
warning: no files found matching 'airflow/providers/google/cloud/example_dags/*.yaml'
warning: no files found matching 'airflow/providers/google/cloud/example_dags/*.sql'
warning: no files found matching 'CHANGELOG.txt'
warning: no files found matching 'README.md'
warning: no previously-included files matching '__pycache__' found anywhere in distribution
warning: no previously-included files matching '*.pyc' found anywhere in distribution
warning: build_py: byte-compiling is disabled, skipping.

This PR will address them

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In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[hussein-awala]

For warning: no files found matching 'README.md', we have a readme on only 3 providers:

• apache beam (not touched since mars 2021)
• neo4j (empty file)
• asana (empty file)

Also I don't see where we are using it, IMHO we can delete include README.md too.