README update with clearer usage instructions and fix gasLimitBoundDivisor

[christianascone]

I just updated README usage section adding some info about input file and output result.
I think it may be a bit clearer now.

[christianascone]

Added a fix for gasLimitBoundDivisor field.
It was previously placed in Ethash object, but according to parity updates it has been moved in params.

[christianascone]

Updated README and json samples according to code updates.
I also edited some sample fields (like extraData and coinbase) setting default values compatible with a geth genesis file which does not provide them.

This way it's easy to convert a simple geth genesis file like this

{
  "config": {
    "chainId": 1,
    "homesteadBlock": 0,
    "eip155Block": 0,
    "eip158Block": 0,
    "byzantiumBlock": 0
  },
  "difficulty": "400",
  "gasLimit": "2000000",
  "alloc": {
    "0000000000000000000000000000000000000001": {
      "balance": "1"
    }
  }
}

and be sure the resulting parity spec will be compatible.