Update Contributing file

[clauBv23]

In the contributing file says that should use pnpm version 7

TypeChain/CONTRIBUTING.md

Lines 7 to 8 in 33879dc

	Heads up: we use pnpm instead of yarn or npm. It might be new for you but I promise it's worth the hassle - it's
	really good at making big monorepos deterministic. Please ensure that you are running pnpm version 7.

meanwhile, in the package.json file, it is forced to be bigger than v8.0.0.

TypeChain/package.json

Lines 5 to 7 in 33879dc

	"engines": {
	"node": ">=14.0.0",
	"pnpm": ">=8.0.0"

It also says that npm version shouldn't be newer than v6

TypeChain/CONTRIBUTING.md

Lines 16 to 17 in 33879dc

	NB: due to a [known issue](https://github.com/npm/cli/issues/3210) in the npm version newer than 6 it is not possible to
	start the commands. A quick fix is to downgrade the npm version `npm install -g npm@6.14.17`.

due to an issue that is already solved