Add tags for task_ids:summarization-* and task_categories:summarization*

[hobson]

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"

[hobson]

Associated community discussion is here.
Paper referenced in the dataset_infos.json is here. 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.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.