Add ofxstatement-co-operative to the known plugins list

[Copilot]

This updates the plugin catalog to include the Co-operative Bank CSV plugin contributed in the issue. The README now lists the plugin alongside other known integrations and adds its repository reference.

• README plugin index

  • Added ofxstatement-co-operative to the Known Plugins table
  • Described it as a Co-operative Bank (UK) plugin

• Link reference

  • Added the corresponding RST link target for the plugin repository:
    https://github.com/JoelColledge/ofxstatement-co-operative

`ofxstatement-co-operative`_      Co-operative Bank (UK)

.. _ofxstatement-co-operative: https://github.com/JoelColledge/ofxstatement-co-operative

[coveralls]

coverage: 95.274%. remained the same — copilot/add-ofxstatement-co-operative-plugin into master