tensorflow-io 0.12.0 release

[yongtang]

With TensorFlow 2.2 release on the horizon it is time for us to release tensorflow-io 0.12.0. Ideally we want to release 0.12.0 against TF 2.1 (before TF 2.2). And, after TF 2.2 is released we will release 0.13.0 against TF 2.2

Our release workflow has been significantly simplified with GitHub Actions. Here is the list of items for 0.12.0:

• Update RELEASE.md/README.md to capture 0.12.0 changes.
• When GitHub Actions complete the build against the master, download the artifact with the name tensorflow-io-release. Uncompress the content and all binaries for release are inside.
• Do minimal sanity test to make sure at least the first section of getting started example code works locally.
• Push all binaries to PyPI.org.

/cc @dmitrievanthony @vlasenkoalexey @BryanCutler @terrytangyuan

Note from tensorflow build/release:

There is a slight change and we will be cutting the r2.2 branch on 3/2/2020 instead of 2/26/2020 as noted earlier.

[terrytangyuan]

We should update the release document as well: https://github.com/tensorflow/community/blob/master/sigs/io/RELEASE.md

[yongtang]

Created a PR for release document:
tensorflow/community#211

[terrytangyuan]

@yongtang I'd be happy to perform the rest of the release steps once RELEASE.md/README.md is updated.

[yongtang]

Release note is being updated on #817.

We have quite a few exciting new features: Windows support, PostgreSQL, MP3, ArrowIOTensor are all very significant. Could highlight them when sending the email out.

[terrytangyuan]

We have just release v0.12.0 here: https://github.com/tensorflow/io/releases/tag/v0.12.0
I've done some preliminary checks locally and all artifacts have been uploaded to PyPI https://pypi.org/project/tensorflow-io/0.12.0/#files. Thanks for everyone's efforts!

[BryanCutler]

Thanks all, great work!