Add draft for data encoding of REVERT

EIPS/eip-draft_revert_data.md

@@ -0,0 +1,48 @@
+## Preamble
+
+    EIP: <to be assigned>
+    Title: Data structure for the REVERT instruction
+    Author: Alex Beregszaszi
+    Type: Standard Track
+    Category: ERC
+    Status: Draft
+    Created: 2017-02-06
+    Requires: (`REVERT` EIP)
+
+## Simple Summary
+
+Suggested data format for the return value of a `REVERT` instruction.
+
+## Abstract
+
+The `REVERT` instruction is able to abort execution, while retaining data to be returned to the caller. This specification defines a suggested format for encoding error messages to be used by this instruction.
+
+## Motivation
+
+The `REVERT` instruction provides a flexible way to return data. It would be beneficial to define a standardised format in order for clients to make use of this data. Clients, such as Mist, could display the error in a user friendly manner.
+
+Potentially in the future, the *Natural Language Specification* could be extended with a list of error codes for each function.
+
+## Specification
+
+The data to be returned via a `REVERT` instruction is defined as an array consisting of two fields:
+- an unsigned number, containing an error code
+- an optional UTF-8 encoded string, containing an error message
+
+This array is encoded using *CBOR* (RFC7049).
+
+## Rationale
+
+The reason CBOR is chosen over RLP is its ability to encode multiple data types and the availability of CBOR libraries in a multitude of languages.
+
+## Backwards Compatibility
+
+This change has no effect on contracts created in the past.
+
+## Test Cases
+
+TBA
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).