Delete instruction to use GOPATH from the CONTRIBUTING's setup tutorial

[joshuabezaleel]

What does this change

It deletes the line on the initial setup from the CONTRIBUTING.md file where it said that porter needs to reside at GOPATH.
This does not needed anymore since go module is introduced and enabled.

I tried this on my local machine to run make build install from a directory outside of the GOPATH and it produces the porter binary. Screenshot is attached below.

What issue does it fix

Closes #1286

Notes for the reviewer

Checklist

• Unit Tests
• Documentation
• Schema (porter.yaml)

[carolynvs]

Thanks for checking that these instructions work on a fresh machine outside of GOPATH!

[carolynvs]

You don't need to change this back but I wanted to share why you will see numbered markdown lists with every item at 1.. The markdown processor handles figuring out the number for you which is helpful when inserting new steps in the middle. You don't need to then update all the other numbers to match.

[joshuabezaleel]

Ah that was handy! Will keep that in mind for future changes 🙂

[carolynvs]

One more thing and then we can merge your pull request. You need to edit your commit and sign it. Here are the instructions on how to do that: https://github.com/getporter/porter/blob/main/CONTRIBUTING.md#signing-your-commits

After you do that, force push your branch back up with git push --force and you should quickly see the "DCO" check pass

[joshuabezaleel]

@carolynvs The thank you should be mine to say for writing a local dev env setup tutorial article that is crystal clear and works like charm! 🙂
Ah sorry that I skipped that part. The check should be good by now 😄

[carolynvs]

I'll update the tutorial to have people install the git commit hook. Thanks for running through it and finding gaps!