inventory: add basic inventory

inventory.md

+# Current events-related inventory
+
+This contains objects that are useful for conferences and that we should keep
+track of. Everything used for events and paid with OpenCollective money must
+be included in there. However, this also contains items that individuals paid
+with their own money but that got shared with the project. The decision of
+whether or not to track these items is at the discretion of the buyers.
+
+- Table cloths:
+  - @craftyguy 1x
+- Stickers:
+  - @PureTryOut
+- Flyers:
+  -
+- Rolls devices wiki page:
+  - @ollieparanoid 1x
+- Phone stand holders:
+  - @lucaweiss 4x