Update contributing doc to include instructions to set up Earthdata Login credentials on a fork so integration tests can run on PRs

[mfisher87]

Should probably be the first step we instruct contributors to do after forking.

[Sherwin-14]

@mfisher87 Would'nt this encompass just a block(having the username and password) with directions?

[mfisher87]

I'm thinking we can maybe make it a sub-bullet of the second bullet on the contributing document?

If we do that, I think we should make "fork" and "setup dev environment" separate bullets, so there would be 4 top-level bullets, and bullet 2 would have the sub-bullet explaining secrets setup. What do you think?

[Sherwin-14]

It's perfect! Let me have a look into that as well once I resolve other ones :)

[mfisher87]

🙇 Thanks, Sherwin!

[Sherwin-14]

Is this one urgent or is it something we can hold for now?

[mfisher87]

I don't feel that it's urgent :) For now I'll un-assign you in case someone else feels like tackling it in the short-term!

[mfisher87]

Will be OBE by #811

[chuckwondo]

OBE by #818

[mfisher87]

Thank you, Chuck! This work will also be useful to icepyx :)

cc @JessicaS11