Add draft skeleton of May 2018 hardfork specification

may-2018-hardfork.md

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+# May 2018 Hardfork Specification
+
+Version Draft 0.2, 2018-02-01
+
+## Summary
+
+When the median time past[1] of the most recent 11 blocks (MTP-11) is greater than or equal to UNIX timestamp 1526400000 Bitcoin Cash will execute a hardfork according to this specification. Starting from the next block these consensus rules changes will take effect:
+
+* Blocksize increase to 32 MB
+* Re-enabling of several opcodes
+
+The following are not consensus changes, but are recommended changes for Bitcoin Cash implementations:
+
+* Automatic replay protection for future hardforks
+* Increase OP_RETURN relay size to 223 total bytes
+
+## Blocksize increase
+
+The blocksize hard capacity limit will be increased to 32MB (32000000 bytes).
+
+## OpCodes
+
+Several opcodes will be re-enabled per [may-2018-opcodes](may-2018-opcodes.md)
+
+## Automatic Replay Protection
+
+When the median time past[1] of the most recent 11 blocks (MTP-11) is greater than or equal to UNIX timestamp 1542300000 Bitcoin Cash full nodes implementing the May 2018 consensus rules SHOULD enforce the following change:
+
+ * Update `forkid`[1] to be equal a non-zero unique constant chosen by each implementation.  It is explicit that this specification intentionally omits a value, as it should be different for each implementation.
+
+This particular consensus rule MUST NOT be implemented by Bitcoin Cash wallet software.
+
+[1] The `forkId` is defined as per the [replay protected sighash](replay-protected-sighash.md) specification.