contract-artifacts: add devdoc to ERC20Token.json

Artifact changes crafted manually: re-generated artifacts from within
@0x/contracts-erc20, and then copied them into @0x/contract-artifacts,
but only committed the changes that added devdoc.

Modified @0x/contracts-erc20/compiler.json to include devdoc in the
hopes that next time contract-artifacts are re-generated en masse, the
devdoc output will just come along for the ride.

Compiling ERC20 TypeScript wrappers after adding devdoc to the artifacts
revealed some inconsistencies in the types.  ethereum-types'
DevdocOutput REQUIRED many fields which are not always present,
depending the devdoc contents itself.  Relaxing the requirement had some
ramifications.

contracts/erc20/compiler.json

@@ -17,7 +17,8 @@
                     "evm.bytecode.object",
                     "evm.bytecode.sourceMap",
                     "evm.deployedBytecode.object",
-                    "evm.deployedBytecode.sourceMap"
+                    "evm.deployedBytecode.sourceMap",
+                    "devdoc"
                 ]
             }
         }