Add a list of instructions to avoid in VM tests.

Author: pirapira

src/VMTestsFiller/README.md

@@ -0,0 +1,21 @@
+VM tests
+========
+
+VM tests test one instance of EVM (which contains one stack of 256-bit words, and one memory space).
+
+Operations accessing the world state should be tested in GeneralStateTests or BlockchainTests instead of in VM tests.
+
+So VM tests should not contain
+
+* `BALANCE`
+* `CREATE`
+* `CREATE2`
+* `CALL`
+* `CALLCODE`
+* `STATICCALL`
+* `DELEGATECALL`
+* `EXTCODESIZE`
+* `EXTCODECOPY`
+* `BLOCKHASH`
+
+Previously, some VM tests contained these, and clients were supposed to implement some mock of the world state (especially, the block hash of a block is supposed to be the hash of the block number).