docs: update README to document TxResponse signers field extension

[Copilot]

• Explore repository structure and locate README.md files
• Verify that TxResponse struct includes the signers field
• Check proto definition showing signers field at line 51 in abci.proto
• Confirm signers field exists in generated Go code (types/abci.pb.go line 69)
• Update main README.md to document the extended TxResponse with signers field
• Test the changes to ensure no build failures
• Verify TxResponse.Signers field works correctly with manual test
• Move TxResponse documentation entry to bottom of the list per feedback
• Final verification and documentation review

Changes made:

• Added bullet point in README.md: "Extended TxResponse to include signers field with the list of transaction signers"
• Moved the entry to the bottom of the fork changes list as requested
• Verified the extension is working correctly through direct testing

Technical verification:

• TxResponse struct properly includes Signers []string field
• Field is defined in proto as repeated string signers = 14;
• All builds and tests pass without issues
• Manual verification confirms the field works as expected

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update the README to include extended signers field in TxResponse </issue_title>
<issue_description>1. We can prob add to the README that TxResponse was extended to include signers

Originally posted by @rootulp in #683 (review)
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #692

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