Update CONTRIBUTING.md with gradle build instructions

[kp-cat]

Gradle build instructions

Update CONTRIBUTING.md with gradle build instructions

• [ ] I have covered the applied changes with automated tests.
• [ ] I have executed the full automated test set against my changes.
• [ ] I have validated my changes against all supported platform versions.
• [x] I have read and accepted the contribution agreement.

[codecov[bot]]

Codecov Report

Merging #9 (bec8c38) into master (baf01de) will decrease coverage by 0.33%. The diff coverage is n/a.

:exclamation: Current head bec8c38 differs from pull request most recent head 243fcbe. Consider uploading reports for the commit 243fcbe to get more accurate results

@@             Coverage Diff              @@
##             master       #9      +/-   ##
============================================
- Coverage     85.78%   85.45%   -0.34%     
  Complexity      237      237              
============================================
  Files            20       20              
  Lines           598      598              
  Branches         86       86              
============================================
- Hits            513      511       -2     
- Misses           56       58       +2     
  Partials         29       29              

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/configcat/AutoPollingPolicy.java	93.54% <0.00%> (-6.46%)	12.00% <0.00%> (ø%)

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
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