docs: improve some READMEs

public/api/users/README.md

@@ -0,0 +1,27 @@
+# /api/users
+
+Interact with users.
+
+## POST /api/users
+
+Create a new user.
+
+```
+{
+	// set the user's username
+	username: string;
+
+	// set the user's password, will be hashed and discarded server-side
+	password?: string;
+
+	// can send an already-hashed password if we want to keep this client-side
+	// should be a base64-encoded SHA-256 hash of the user's client-entered pw
+	password_hash?: string;
+}
+```
+
+## GET /api/users
+
+Get users. (admin)
+
+TODO: document this