[No Jira] Update README to have correct staging environment

[zweatshirt]

Description

• The README does not have the correct staging environment.
• The incorrect staging environment link leads to confusion for new developers.

Testing

Read the REAMDE

Checklist:

• I have given my PR a title with the format "EP-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels (Add the label "On Staging" to get the branch automatically merged into staging)
• I have requested a review from another person on the project
• I have checked that stage-branch-merger successfully merged my branch to staging or manually merged it myself
• I have tested my changes in staging for regular checkout
• I have tested my changes in staging for branded checkout

[stage-branch-merger]

I see you added the "On Staging" label, I'll get this merged to the staging branch!

[dr-bizz]

This needs to stay as it's for the branded-checkout, but you're right that we need to improve the developer instructions. Perhaps add a section for getting to staging near where we have the dev setup instructions.

Here it is in use: https://give-stage-static.cru.org/give.min.css