Credit Eric Litman in the README license section

[arjitj2]

Closes #93.

Name and link Eric Litman's Open Pstack adaptations in the README License section alongside Lauren Tan and Michael Denyer. The wording follows NOTICE.md. Eric's upstream LICENSE also retains Lauren's copyright notice, which is unchanged here.

Validation:

• Documentation checker, 16 documentation tests, and whitespace checks pass.
• Only README.md changes. The packaged plugin tree is identical to main at cfc7c8e, so no parent installation is affected.
• Read both repositories' live LICENSE files and the current attribution notice.
• The identical package passed the full test suite, strict typecheck, static invariants and plugin validation during the preceding release work. GitHub CI repeats the required checks for this head.

Documentation owner updated: README.md, License section.

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[Copilot]

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[arjitj2]

Independent maintenance verification: PASS for 5b98e5a against cfc7c8e.

The only change is the README License paragraph. Eric's credit agrees with NOTICE.md; Lauren and Michael remain credited. The plugin tree is unchanged at 8bf32690c3e41d0b047d80a198a0d73209d2b140. Documentation and whitespace checks pass in a clean detached checkout. No findings.