Mnemonic & Private keys are different - Please update readme

[idiom-bytes]

Problem

The readme communicates 2 different concepts as if they're the same.

Set a private key and export it
export MNEMONIC="XXXX"

Mnemonics are the 12-word random keys required to generate private keys. In barge this is preconfigured so you always get the same private keys.

export GANACHE_MNEMONIC=${GANACHE_MNEMONIC:-"taxi music thumb unique chat sand crew more leg another off lamp"}

private-key is the 0x hash associated with a specific EOA

0xfd5c1ccea015b6d663618850824154a3b3fb2882c46cefb05b9a93fea8c3d215

DoD:

• [ ] Clean up mnemonic/private key in README so it's obvious to the user
• [ ] Perhaps provide an example of what you're looking for

  export MNEMONIC="taxi music thumb..."

  or

  export PRIVATE_KEY="0xfd5c1ccea015b6d66...."

[mihaisc]

Thanks for noticing. Change the var to PRIVATE_KEY since we are actually using only the private key. For mnemonic we should use fromMnemonic but doesn't make sense to implement it right now