docs: add General Availability banner to README (#507)

* docs: add General Availability banner to README

As of 1.0.0 release, BigQuery DataFrames is available as a [Generally Available](https://github.com/googleapis/google-cloud-python/blob/main/README.rst#general-availability) Google Cloud product.

* update release status

README.rst

@@ -1,6 +1,8 @@
 BigQuery DataFrames
 ===================
 
+|GA| |pypi| |versions|
+
 BigQuery DataFrames provides a Pythonic DataFrame and machine learning (ML) API
 powered by the BigQuery engine.
 
@@ -10,6 +12,12 @@ powered by the BigQuery engine.
 BigQuery DataFrames is an open-source package. You can run
 ``pip install --upgrade bigframes`` to install the latest version.
 
+.. |GA| image:: https://img.shields.io/badge/support-GA-gold.svg
+   :target: https://github.com/googleapis/google-cloud-python/blob/main/README.rst#general-availability
+.. |pypi| image:: https://img.shields.io/pypi/v/bigframes.svg
+   :target: https://pypi.org/project/bigframes/
+.. |versions| image:: https://img.shields.io/pypi/pyversions/bigframes.svg
+   :target: https://pypi.org/project/bigframes/
 
 Documentation
 -------------

setup.py

@@ -30,7 +30,7 @@
 # 'Development Status :: 3 - Alpha'
 # 'Development Status :: 4 - Beta'
 # 'Development Status :: 5 - Production/Stable'
-release_status = "Development Status :: 3 - Alpha"
+release_status = "Development Status :: 5 - Production/Stable"
 dependencies = [
     # please keep these in sync with the minimum versions in testing/constraints-3.9.txt
     "cloudpickle >= 2.0.0",