Add readme about identifiers

uncloud_django_based/uncloud/doc/README-identifiers.org

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+* Identifiers
+** Problem description
+   Identifiers can be integers, strings or other objects. They should
+   be unique.
+** Approach 1: integers
+   Integers are somewhat easy to remember, but also include
+   predictable growth, which might allow access to guessed hacking
+   (obivously proper permissions should prevent this).
+** Approach 2: random uuids
+   UUIDs are 128 bit integers. Python supports uuid.uuid4() for random
+   uuids.
+** Approach 3: IPv6 addresses
+   uncloud heavily depends on IPv6 in the first place. uncloud could
+   use a /48 to identify all objects. Objects that have IPv6 addresses
+   on their own, don't need to draw from the system /48.
+*** Possible Subnetworks
+    Assuming uncloud uses a /48 to represent all resources.
+
+    | Network         | Name            | Description                                  |
+    |-----------------+-----------------+----------------------------------------------|
+    | 2001:db8::/48   | uncloud network | All identifiers drawn from here              |
+    | 2001:db8:1::/64 | VM network      | Every VM has an IPv6 address in this network |
+    | 2001:db8:2::/64 | Bill network    | Every bill has an IPv6 address               |
+    | 2001:db8:3::/64 | Order network   | Every order has an IPv6 address              |
+    | 2001:db8:5::/64 | Product network | Every product (?) has an IPv6 address        |
+    | 2001:db8:4::/64 | Disk network    | Every disk is identified                     |
+
+*** Tests
+    [15:47:37] black3.place6:~# rbd create -s 10G ssd/2a0a:e5c0:1::8