gh cli to CONTRIBUTING.md

# Add `gh` cli to the CONTRIBUTING.md

Add the `gh` cli as on option to every section that has `git` in the CONTRIBUTING.md.

for example:
```sh
git clone https://github.com/adamkali/dotnvim.git
```

the following should be added as well:
```sh
gh repo clone adamkali/dotnvim
```