[Backport 2.3] Add Forum link in Index Management plugin README.md

[opensearch-trigger-bot]

Backport 79214a2 from #499

[codecov-commenter]

Codecov Report

Merging #554 (5f09a5f) into 2.3 (9af3b15) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                2.3     #554   +/-   ##
=========================================
  Coverage     75.79%   75.79%           
- Complexity     2476     2480    +4     
=========================================
  Files           315      315           
  Lines         14511    14511           
  Branches       2245     2245           
=========================================
  Hits          10999    10999           
- Misses         2255     2256    +1     
+ Partials       1257     1256    -1     

Impacted Files	Coverage Δ
.../rollup/action/start/TransportStartRollupAction.kt	71.76% <0.00%> (-3.53%)	⬇️
.../action/explain/TransportExplainTransformAction.kt	70.78% <0.00%> (-2.25%)	⬇️
...nt/rollup/action/stop/TransportStopRollupAction.kt	76.47% <0.00%> (-1.18%)	⬇️
...nt/indexstatemanagement/ManagedIndexCoordinator.kt	69.00% <0.00%> (-0.30%)	⬇️
...earch/indexmanagement/transform/model/Transform.kt	85.47% <0.00%> (ø)
...ndexstatemanagement/IndexStateManagementHistory.kt	80.00% <0.00%> (+1.37%)	⬆️
...gement/indexstatemanagement/action/ShrinkAction.kt	72.05% <0.00%> (+1.47%)	⬆️
...nt/indexstatemanagement/model/destination/Chime.kt	77.77% <0.00%> (+22.22%)	⬆️

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