Add links to changelog

Author: tom-andersen

firebase-firestore/CHANGELOG.md

@@ -1,8 +1,8 @@
 # Unreleased
 - [fixed] Fix FAILED_PRECONDITION when writing to a deleted document in a transaction.
   ([Issue #5871](//github.com/firebase/firebase-android-sdk/issues/5871))
-- [fixed] Fixed Firestore failing to raise initial snapshot from empty local cache result.
-- [fixed] Removed invalid suggestions to use `GenericTypeIndicator` from error messages.
+- [fixed] Fixed Firestore failing to raise initial snapshot from empty local cache result. ([PR](https://github.com/firebase/firebase-android-sdk/pull/4207))
+- [fixed] Removed invalid suggestions to use `GenericTypeIndicator` from error messages. ([Issue 222](https://github.com/firebase/firebase-android-sdk/issues/222))
 - [changed] Updated dependency of `io.grpc.*` to its latest
   version (v1.50.2).