Improve README on editing PR

[Ryuno-Ki]

Today, several contributors asked how to edit files on a PR.

For this, you will go to the files tab of the PR, pick the file you want, click the three dots menu in the upper right corner and go to Edit File. The changes should result in a new commit.

Are you okay with me improving the README with this instruction?

[teoli2003]

Consider: you will go -> go and should -> will

(I'm not a native speaker so it may be better your way)

[Ryuno-Ki]

I'm not a native speaker either.
The above was more like a suggestion on the direction I want to go.
The exact phrasing will be debated in the PR itself, I guess.

[teoli2003]

Sounds good to me!

[Ryuno-Ki]

Great! Okay, will cook a PR then.

[Rumyra]

Feel free to assign me as a reviewer on the PR @Ryuno-Ki 👍

[Ryuno-Ki]

Will do. It's a bit busy for me, but I have this on my agenda.

[Ryuno-Ki]

Finally got around to fill #7563

[pepelsbey]

Closing as answered/settled mdn/content#7563