Add CI and Codecov badges in README

[kaijchen]

What changes were proposed in this pull request?

Add CI and Codecov badges in README.

Why are the changes needed?

It's easier to see the CI and Codecov result.
And you can click the badges for details.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

See: https://github.com/kaijchen/incubator-uniffle/blob/badges/README.md

[codecov-commenter]

Codecov Report

Merging #20 (79a6cc6) into master (eb15611) will decrease coverage by 0.89%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #20      +/-   ##
============================================
- Coverage     56.27%   55.37%   -0.90%     
+ Complexity     1168     1097      -71     
============================================
  Files           152      143       -9     
  Lines          8397     8030     -367     
  Branches        812      783      -29     
============================================
- Hits           4725     4447     -278     
+ Misses         3408     3329      -79     
+ Partials        264      254      -10     

Impacted Files	Coverage Δ
...er/impl/UploadedStorageHdfsShuffleReadHandler.java	76.31% <0.00%> (-5.27%)	⬇️
.../java/org/apache/uniffle/server/ShuffleServer.java	68.14% <0.00%> (-4.43%)	⬇️
...e/uniffle/server/storage/SingleStorageManager.java	65.57% <0.00%> (-1.64%)	⬇️
...e/spark/shuffle/reader/RssShuffleDataIterator.java
.../org/apache/spark/shuffle/writer/WriterBuffer.java
...che/spark/shuffle/writer/BufferManagerOptions.java
...ava/org/apache/spark/shuffle/RssShuffleHandle.java
...org/apache/spark/shuffle/writer/AddBlockEvent.java
...k/shuffle/writer/WrappedByteArrayOutputStream.java
...org/apache/spark/shuffle/RssSparkShuffleUtils.java
... and 2 more

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[jerqi]

LGTM