Add readme for billing

doc/README-billing.org

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+* How to handle billing in general
+** Orders
+   - Orders are the heart of uncloud billing
+   - Have a starting date
+   - Have an ending date
+*** Statuses
+    - CREATING/PREPARING
+    - INACTIVE (?)
+    - TO_BILL / NOT_TO_BILL
+*** Updating status of orders
+    - If has succeeding order and billing date is last month -> set inactive
+** Bills
+   - Are always for a month
+   - Can be preliminary
+*** Which orders to include
+    - Not the cancelled ones / not active ones
+    -
+** Flows / Approach
+*** Finding all orders for a bill
+    - Get all orders, state != NOT_TO_BILL; for each order do:
+      - is it a one time order?
+        - has it a bill assigned?
+          - yes: set to NOT_TO_BILL
+          - no:
+            - get_or_create_bill_for_this_month
+            - assign bill to this order
+            - set to NOT_TO_BILL
+
+    - First of month
+    - Last of month