doc: update README

[smyrick]

Fixes #125

• Add javadoc badge https://javadoc.io/doc/com.expedia/graphql-kotlin/0.0.29
• Update maven central badge to the recommended one on the side bar here: https://search.maven.org/artifact/com.expedia/graphql-kotlin
• Update README code

[codecov-io]

Codecov Report

Merging #126 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #126   +/-   ##
=========================================
  Coverage     90.49%   90.49%           
  Complexity      157      157           
=========================================
  Files            47       47           
  Lines           505      505           
  Branches        112      112           
=========================================
  Hits            457      457           
  Misses           10       10           
  Partials         38       38

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