Add "create documentation" to add_tasks.md

[neubig]

There is a guide on how to add new tasks: https://github.com/neulab/ExplainaBoard/blob/main/docs/add_new_tasks.md

However it doesn't mention adding documentation, and it should. Here are some places where tasks should be added:

• https://github.com/neulab/ExplainaBoard/blob/main/docs/cli_interface.md
• https://github.com/neulab/ExplainaBoard/pull/553

[qjiang002]

Resolved in PR #553