add basic README

README.md

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+# spam management for forgejo
+
+## Usage
+
+- create an API token for your admin account, and write it in an `api_token`
+  file at the root of the repo
+- remove `model.json` if you want to start with no pre-existing model of what is
+  spam or not. Or keep it to use the current classifier. The file gets updated
+  when using the tool: the classifier learns from spam/legit decisions and
+  should get progressively better at identifying spam.
+- run: `cargo run`
+- classify users as spam/not spam. Right now the classification is stored
+  locally in `db.json`, no concrete action is taken. (Ultimately we will want to
+  lock/delete accounts, etc.)