feat: update merge packages info

[rin-st]

For me it wasn't clear how versions in merged package.json files are defined. From readme, I thought that always version from second package.json is taken. So I updated readme to make it clear for other devs.

Source https://github.com/zppack/merge-packages/blob/master/src/index.js#L104

If you want to test examples, you can add this to your first templates/base/packages/nextjs/package.json deps

    "a": "~1.2.3",
    "b": "^1.0.0",

and create new package.json at `templates/solidity-frameworks/hardhat/packages/nextjs/package.json

{
  "dependencies": {
    "a": "^1.0.0",
    "b": ">=1.2.0 <1.3.0"
  }
}

run

yarn cli -s hardhat --skip test-packages-merge

and check resulting nextjs package