doc(archive/100-theorems-list): Update README.md (#2750)

Making the 100.yaml file more discoverable.

Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>

archive/100-theorems-list/README.md

@@ -1 +1,6 @@
-for theorems on the [100 theorems list](https://www.cs.ru.nl/~freek/100/) which don't occur naturally in mathlib
+# "Formalizing 100 Theorems" in Lean
+
+In this folder, we keep proofs of theorems on Freek Wiedijk's [100 theorems list](https://www.cs.ru.nl/~freek/100/) which don't fit naturally elsewhere in mathlib or in other Lean repositories.
+
+See [this page](https://leanprover-community.github.io/100.html) for more information about theorems from the list above which have been formalized in Lean. If you prove a new theorem from that list, you should add the appropriate data to this file:
+https://github.com/leanprover-community/leanprover-community.github.io/blob/newsite/data/100.yaml