add_tasks.md is out of date

[tetsuok]

It seems add_tasks.md is out of date. add_tasks.md mentions tasks.py in three places below:

• https://github.com/neulab/ExplainaBoard/blame/fcedd5d7aab172b943c6b0025685b09744f149fd/docs/add_new_tasks.md#L6
• https://github.com/neulab/ExplainaBoard/blame/fcedd5d7aab172b943c6b0025685b09744f149fd/docs/add_new_tasks.md#L12
• https://github.com/neulab/ExplainaBoard/blame/fcedd5d7aab172b943c6b0025685b09744f149fd/docs/add_new_tasks.md#L133

but the Python script was removed in #373. add_tasks.md needs to be updated properly.