Add tags for task_ids:summarization-* and task_categories:summarization*
Associated community discussion is [here](https://huggingface.co/datasets/aeslc/discussions/1).
Paper referenced in the `dataset_infos.json` is [here](https://arxiv.org/pdf/1906.03497.pdf). It mentions the  _email-subject-generation_ task, which is not a tag mentioned in any other dataset so it was not added in this pull request. The _summarization_ task is mentioned as a related task. _The documentation is not available anymore as the PR was closed or merged._
yaml header at top of README.md file was edited to add task tags because I couldn't find the existing tags in the json
separate Pull Request will modify dataset_infos.json to add these tags

The Enron dataset (dataset id aeslc) is only tagged with:

    arxiv:1906.03497'
    languages:en
    pretty_name:AESLC

Using the email subject_line field as a label or target variable it possible to create models for the following task_ids (in order of relevance):

    'task_ids:summarization'
    'task_ids:summarization-other-conversations-summarization'
    "task_ids:other-other-query-based-multi-document-summarization"
    'task_ids:summarization-other-aspect-based-summarization'
    'task_ids:summarization--other-headline-generation'

The subject might also be used for the task_category "task_categories:summarization"

E-mail chains might be used for the task category "task_categories:dialogue-system"