doc: add package readme link back to proposal readme

EisenbergEffect · EisenbergEffect · commit 8ea158302d7c · 2024-03-27T13:32:38.000-04:00
diff --git a/packages/signal-polyfill/readme.md b/packages/signal-polyfill/readme.md
@@ -1,6 +1,6 @@
 # Signal Polyfill
 
-A "signal" is a proposed first-class JavaScript data type that enables one-way data flow through cells of state or computations derived from other state/computations.
+A "signal" is [a proposed first-class JavaScript data type](../../README.md) that enables one-way data flow through cells of state or computations derived from other state/computations.
 
 This is a polyfill for the `Signal` API.
 
