[REST] adding basic documentation

doc/REST-interface.md

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+Unauthenticated REST Interface
+==============================
+
+The REST API can be enabled with the `-rest` option.
+
+Supported API
+-------------
+`GET /rest/tx/TX-HASH.{bin|hex|json}`
+
+Given a transaction hash,
+Returns a transaction, in binary, hex-encoded binary or JSON formats.
+
+`GET /rest/block/BLOCK-HASH.{bin|hex|json}`
+
+Given a block hash,
+Returns a block, in binary, hex-encoded binary or JSON formats.
+
+The HTTP request and response are both handled entirely in-memory, thus making maximum memory usage at least 2.66MB (1 MB max block, plus hex encoding) per request.
+
+For full TX query capability, one must enable the transaction index via "txindex=1" command line / configuration option.
+
+Risks
+-------------
+Running a webbrowser on the same node with a REST enabled bitcoind can be a risk. Accessing prepared XSS websites could read out tx/block data of your node by placing links like `<script src="http://127.0.0.1:1234/tx/json/1234567890">` which might break the nodes privacy.