Add README

fabric-tag-api-v1/README.md

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+# Fabric Tag API (v1)
+
+This module contains APIs for working with data pack tags.
+
+## Tag aliases
+
+*Tag alias groups* merge tags that refer to the same set of registry entries.
+The contained tags will be linked together and get the combined set of entries
+of all the aliased tags in a group.
+
+Tag alias groups can be defined in data packs in the `data/<mod namespace>/fabric/tag_alias/<registry>`
+directory. `<registry>` is the path of the registry's ID, prefixed with `<registry's namespace>/` if it's
+not `minecraft`.
+
+The JSON format of tag alias groups is an object with a `tags` list containing plain tag IDs.
+
+See the module javadoc for more information about tag aliases.