Add Forum link in SQL plugin README.md

[cwillum]

Signed-off-by: cwillum cwmmoore@amazon.com

Description

Added Forum header section that includes verbiage and a link to the OpenSearch Forum page for the SQL plugin.

Issues Resolved

This more closely corresponds to the information found in the README.md file in all of the other plugins bundled with OpenSearch.

This fixes the issue for the SQL plugin in #921.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov-commenter]

Codecov Report

Merging #809 (17783a6) into main (ddb3deb) will increase coverage by 32.05%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##               main     #809       +/-   ##
=============================================
+ Coverage     62.76%   94.82%   +32.05%     
- Complexity        0     2912     +2912     
=============================================
  Files            10      291      +281     
  Lines           658     7783     +7125     
  Branches        118      566      +448     
=============================================
+ Hits            413     7380     +6967     
- Misses          192      349      +157     
- Partials         53       54        +1     

Flag	Coverage Δ
query-workbench	62.76% <ø> (ø)
sql-engine	97.78% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...g/opensearch/sql/expression/LiteralExpression.java	100.00% <0.00%> (ø)
...arch/storage/script/filter/lucene/LuceneQuery.java	100.00% <0.00%> (ø)
...h/sql/expression/conditional/cases/WhenClause.java	100.00% <0.00%> (ø)
...ch/sql/opensearch/client/OpenSearchNodeClient.java	100.00% <0.00%> (ø)
...ression/operator/arthmetic/ArithmeticFunction.java	100.00% <0.00%> (ø)
...ch/planner/logical/rule/MergeSortAndIndexScan.java	100.00% <0.00%> (ø)
...nsearch/data/value/OpenSearchExprValueFactory.java	100.00% <0.00%> (ø)
...arch/sql/expression/aggregation/AvgAggregator.java	100.00% <0.00%> (ø)
...org/opensearch/sql/analysis/HighlightAnalyzer.java	100.00% <0.00%> (ø)
...nsearch/response/error/OpenSearchErrorMessage.java	100.00% <0.00%> (ø)
... and 271 more

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[dai-chen]

Thanks for the changes!

[cwillum]

@opensearch-project/sql Thanks for the review. And sorry to break the SQL Java CI check. When that's resolved and the reviews are complete, could someone merge this PR? I don't have that permission in this repo. Thanks.