Add badges to AD

[yizheliu-amazon]

Issue #, if available:

Description of changes:
Add badges to README

Similar to AD Kibana: opendistro-for-elasticsearch/anomaly-detection-kibana-plugin#262

Test example branch: https://github.com/opendistro-for-elasticsearch/anomaly-detection/tree/badges-test

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@68429a8). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #199   +/-   ##
=========================================
  Coverage          ?   71.27%           
  Complexity        ?     1274           
=========================================
  Files             ?      134           
  Lines             ?     5292           
  Branches          ?      463           
=========================================
  Hits              ?     3772           
  Misses            ?     1364           
  Partials          ?      156           

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