adding documentation

docs/how_to_use_the_api_get.rst

@@ -573,6 +573,19 @@ https://github.com/ucoin-io/ucoin/blob/master/doc/HTTP_API.md#blockchainwithtx
       print(blockchainWithTx)
 
 
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+
+``/blockchain/branches`` :
+
+https://github.com/ucoin-io/ucoin/blob/master/doc/HTTP_API.md#blockchainbranches
+
+.. code-block:: python
+
+  # ----------- GET current branches of the node. -----------
+
+  # Not yet implemented in the Python API : /blockchain/branches
+
+
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 ``/tx/sources/[pubkey]`` :