[Backport 2.x] Add Forum link in Index Management plugin README.md

[opensearch-trigger-bot]

Backport 79214a2 from #499

[codecov-commenter]

Codecov Report

Merging #555 (01cfb7c) into 2.x (12917cd) will increase coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x     #555      +/-   ##
============================================
+ Coverage     75.82%   75.94%   +0.11%     
- Complexity     2488     2489       +1     
============================================
  Files           316      316              
  Lines         14547    14547              
  Branches       2248     2248              
============================================
+ Hits          11030    11047      +17     
+ Misses         2258     2240      -18     
- Partials       1259     1260       +1     

Impacted Files	Coverage Δ
...gement/transform/model/ContinuousTransformStats.kt	70.37% <0.00%> (-3.71%)	⬇️
.../rollup/action/start/TransportStartRollupAction.kt	71.76% <0.00%> (-3.53%)	⬇️
...management/indexstatemanagement/MetadataService.kt	62.68% <0.00%> (-2.24%)	⬇️
...nt/rollup/action/stop/TransportStopRollupAction.kt	76.47% <0.00%> (-1.18%)	⬇️
.../opensearch/indexmanagement/rollup/model/Rollup.kt	85.58% <0.00%> (-0.47%)	⬇️
...nt/indexstatemanagement/ManagedIndexCoordinator.kt	69.29% <0.00%> (ø)
...earch/indexmanagement/transform/model/Transform.kt	85.89% <0.00%> (+0.42%)	⬆️
...gement/indexstatemanagement/action/ShrinkAction.kt	72.05% <0.00%> (+2.94%)	⬆️
...anagement/indexstatemanagement/model/Transition.kt	65.75% <0.00%> (+4.10%)	⬆️
...statemanagement/model/destination/CustomWebhook.kt	95.71% <0.00%> (+28.57%)	⬆️

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