doc of decentralized permanent object storage

STORAGE.md

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+# Mixin Object Storage
+
+Mixin Kernel offers a permanent and decentralized object storage solution in the distributed ledger.
+
+## Store
+
+An object storage transaction is a normal Mixin Kernel transaction that supports a maximum 4MB arbitrary data in the transaction extra. The transaction must comply with the following rules:
+
+1. The transaction asset must be XIN.
+2. The first output must be any 64/1 script keys, i.e. the output value is slashed.
+3. The first output value must be no lower than `0.0001*(len(extra)/1024+1)`.
+
+Then the data will be permanently stored in the decentralized Mixin Network.
+
+## Retrieve
+
+To retrieve the stored object, just use the `gettransaction` RPC call to any Mixin Kernel nodes. However we provide a simpler public HTTP API.
+
+```
+GET https://kernel.mixin.dev/objects/TX-HASH
+```
+
+This will respond the extra data of the transaction `TX-HASH`. For now the `Content-Type` could be `application/json`, `text/plain` and `application/octet-stream`.
+
+If the extra is a JSON dictionary, it's possible to query each field at the first level:
+
+```
+GET https://kernel.mixin.dev/objectx/TX-HASH/FIELD
+```
+
+If the field data is a valid data URI scheme, then the parsed media type value, e.g. `image/webp`, will be used to set the HTTP response `Content-Type` header.
+
+The host `kernel.mixin.dev` used in the sample could be replaced with any Mixin Kernel node RPC host.