add README

data/features/README.md

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+## Feature-based versioning
+
+Feature-based versioning allows us to define and control the versions of an arbitrarily named "feature" in one place.
+
+## How it works
+
+Add a new YAML file with the feature name you want to use in this directory. For a feature named `meow`, that would be `data/features/meow.yml`.
+
+Add a `versions` block to the YML file with the short names of the versions the feature is available in. For example:
+
+```yaml
+versions:
+  fpt: '*'
+  ghes: '>3.1'
+  ghae: '*'
+```
+
+The format and allowed values are the same as the [frontmatter versions property](/content#versions).
+
+### Liquid conditionals
+
+Now you can use `{% if meow %} ... {% endif %}` in content files! Note this is the `if` tag, not the new `ifversion` tag.
+
+### Frontmatter
+
+You can also use the feature in frontmatter in content files:
+
+```yaml
+versions:
+  fpt: '*'
+  ghes: '>3.1'
+  feature: 'meow'
+```
+
+If you want a content file to apply to more than one feature, you can do this:
+
+```yaml
+versions:
+  fpt: '*'
+  ghes: '>3.1'
+  feature: ['meow', 'blorp']
+```
+
+## Schema enforcement
+
+The schema for validating the feature versioning lives in [`tests/helpers/schemas/feature-versions.js`](tests/helpers/schemas/feature-versions.js) and is exercised by [`tests/content/lint-files.js`](tests/content/lint-files.js).
+
+## Script to remove feature tags
+
+TBD!