docs(README): add superagent-aws-sign to plugin list

[HannesOberreiter]

The prefix contains an organisation scope, hope thats ok (because I have written the plugin in my office hours).

Cheers
Hannes

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.