Fix README Build Status

[zachgk]

Description

We display a build status at the top of the README between the mxnet logo and the Portable, Efficient, Scalable graphics. It is currently broken due to splitting the CI validation into multiple steps. This PR displays a build status for each of the steps as I don't know if Jenkins has a combined view.

Old Broken - https://github.com/apache/incubator-mxnet/blob/2a55cd7d1801e6ed801057cac9018b707fe3e27f/README.md
New 1 - https://github.com/zachgk/incubator-mxnet/blob/c62becbc73d74ae066fb1857a7aa3140a38bfb07/README.md
New 2 - https://github.com/zachgk/incubator-mxnet/blob/de80f2a0e27fc7af7489d0cb8fd2d3f190854d89/README.md

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[aaronmarkham]

Check the preview... each button doesn't have any text with it, so it just a bunch of unlabeled badges.
https://github.com/apache/incubator-mxnet/blob/c62becbc73d74ae066fb1857a7aa3140a38bfb07/README.md

[zachgk]

@aaronmarkham Ok. I figured out how to label the badges so the current version is https://github.com/zachgk/incubator-mxnet/blob/de80f2a0e27fc7af7489d0cb8fd2d3f190854d89/README.md.