[meta] document the workflows

Author: tabatkins

.github/README.md

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+Types of Workflows
+==================
+
+* `workflows/build-specs.yml` - builds the entire repository of Editor's Drafts, along with a summary page. Also builds any `.issues` Issues-List files, and Markdown files. Deploys all the results to the repo's gh-pages
+
+* `workflows/SPEC-NAME-1.yml` - each of these auto-publish *one* spec in the repo. Do not modify them by hand; they are auto-generated by [`generate-auto-publish-workflows.py`](https://github.com/w3c/csswg-drafts/blob/main/.github/generate-auto-publish-workflows.py) and [`auto-publish-template.yml`](https://github.com/w3c/csswg-drafts/blob/main/.github/auto-publish-template.yml). Instead, update and run this `.py` file and commit the results.