message: little document update clarify steps to be able to run the example
diff: git clone git@github.com:maticnetwork/matic.js.git
  ```
  +
  +
  +### Edit config.js
  +```
  +PRIVATE_KEY: '<paste your private key here>', // A sample private key prefix with `0x`
  +FROM_ADDRESS: '<paste address belonging to private key here>',// Your address
  +```
  +Update following lines with your wallet address and key.
  +Be careful with your private key, Use a key your are comfortable with for development purpose.
  +
  ### Install and run:
  ```bash
  cd examples/node
  npm install
  node balance-of-ERC20.js
  \ No newline at end of file
  -```
  