Add README for ready-to-use packs

packs/README.md

+# packs
+
+This directory contains some ready-to-use stickerpack metadata. Currently
+it's just all the packs imported from Scalar (the default integration manager).
+
+To use these, copy the packs you want to `web/packs/`, then edit
+`web/packs/index.json` to include the file names you copied in the `packs`
+array. The index.json file should look something like this:
+
+```json
+{
+  "homeserver_url": "https://example.com",
+  "packs": [
+    "your_telegram_imported_pack.json",
+    "another_telegram_imported_pack.json",
+    "scalar-rabbit.json",
+    "scalar-loading_artist.json"
+  }
+}
+```