add note to payments welcome page

[jkup]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Fix #4067

Auditors: @diracdeltas @bradleyrichter

Test Plan:

Make sure the note about Brave Payments is on the welcome page as the second to last paragraph.

[bbondy]

++