docs(backers): list the first sponsor

BACKERS.md still read "Be the first." in every tier while the README
claimed "Every sponsor is listed in BACKERS.md" — the same promise
CONTRIBUTING.md uses to reserve the README for people funding the work.
That promise had no backing.

Also states which monthly amount maps to which tier. The sponsors page
sells $5/$15/$50/$100/$250, none of them named Gold/Silver/Bronze, so
nothing said where a given sponsor belongs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

BACKERS.md

@@ -6,6 +6,9 @@ Thank you to everyone who sponsors my open-source work. Your support funds PR re
 
 ---
 
+Tiers follow the monthly amount on the [sponsors page](https://github.com/sponsors/virgiliojr94):
+Gold $100+ · Silver $50–99 · Bronze $15–49 · Backers $5–14 and one-time.
+
 ## 🥇 Gold Sponsors
 
 _Be the first._
@@ -20,7 +23,7 @@ _Be the first._
 
 ## 💖 Backers
 
-_Be the first._
+- **[Gui Santos (@guiofsaints)](https://github.com/guiofsaints)** — the first person to sponsor this work.
 
 ---