Added release note regarding hex field in tx JSON from getblock
Added release note regarding hex field in tx JSON from getblock

Summary:
Fixes https://github.com/Bitcoin-ABC/bitcoin-abc/issues/272
Verifies that the backport referenced in T500 actually fixes the above issue.

Test Plan:
Verified on testnet:
```
    {
      "txid": "f8a0e27f8466d4f291fb30b4439a38711700d1fe007b99e66a1f09b340f8a62b",
      "hash": "f8a0e27f8466d4f291fb30b4439a38711700d1fe007b99e66a1f09b340f8a62b",
      "version": 1,
      "size": 226,
      "locktime": 0,
      "vin": [
        {
          "txid": "4e71ca3f45a8761c3db4f7e6b59052a645d2b25a4c2399acf9ae53695c73844a",
          "vout": 1,
          "scriptSig": {
            "asm": "3045022100f310430c619590da883c96f5e20c5784d466979af0603f5c608bb3190e0f1ed5022036dbafb40b49cf3f3d1e5c5b319acb583a2ebad0c92f2eca0fef7b0c5a3932f8[ALL|FORKID] 02c202701835c99b3e977acd51afc478325cc981bb8456be585307a8382cd7f2c7",
            "hex": "483045022100f310430c619590da883c96f5e20c5784d466979af0603f5c608bb3190e0f1ed5022036dbafb40b49cf3f3d1e5c5b319acb583a2ebad0c92f2eca0fef7b0c5a3932f8412102c202701835c99b3e977acd51afc478325cc981bb8456be585307a8382cd7f2c7"
          },
          "sequence": 4294967295
        }
      ],
      "vout": [
        {
          "value": 0.00002,
          "n": 0,
          "scriptPubKey": {
            "asm": "OP_DUP OP_HASH160 32b57f34861bcbe33a701be9ac3a50288fbc0a3d OP_EQUALVERIFY OP_CHECKSIG",
            "hex": "76a91432b57f34861bcbe33a701be9ac3a50288fbc0a3d88ac",
            "reqSigs": 1,
            "type": "pubkeyhash",
            "addresses": [
              "bchtest:qqet2le5scduhce6wqd7ntp62q5gl0q285mw88lsca"
            ]
          }
        },
        {
          "value": 3.53384068,
          "n": 1,
          "scriptPubKey": {
            "asm": "OP_DUP OP_HASH160 86101da37d5ed4800ca902fdad8aaf99ec7df250 OP_EQUALVERIFY OP_CHECKSIG",
            "hex": "76a91486101da37d5ed4800ca902fdad8aaf99ec7df25088ac",
            "reqSigs": 1,
            "type": "pubkeyhash",
            "addresses": [
              "bchtest:qzrpq8dr040dfqqv4yp0mtv247v7cl0j2qgxpnsevr"
            ]
          }
        }
      ],
      "hex": "01000000014a84735c6953aef9ac99234c5ab2d245a65290b5e6f7b43d1c76a8453fca714e010000006b483045022100f310430c619590da883c96f5e20c5784d466979af0603f5c608bb3190e0f1ed5022036dbafb40b49cf3f3d1e5c5b319acb583a2ebad0c92f2eca0fef7b0c5a3932f8412102c202701835c99b3e977acd51afc478325cc981bb8456be585307a8382cd7f2c7ffffffff02d0070000000000001976a91432b57f34861bcbe33a701be9ac3a50288fbc0a3d88ac84361015000000001976a91486101da37d5ed4800ca902fdad8aaf99ec7df25088ac00000000"
    }
```

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2513

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