Update changelog in prep for release (firebase#39)

Author: rsgowman

firebase-firestore/CHANGELOG.md

@@ -1,11 +1,14 @@
-# Unreleased
+# 17.1.1
 - [fixed] Fixed an issue where the first `get()` call made after being offline
  could incorrectly return cached data without attempting to reach the backend.
 - [changed] Changed `get()` to only make 1 attempt to reach the backend before
  returning cached data, potentially reducing delays while offline. Previously
  it would make 2 attempts, to work around a backend bug.
 - [fixed] Fixed an issue that caused us to drop empty objects from calls to
  `set(..., SetOptions.merge())`.
+- [fixed] Ensure printf style templates are compile time constants. Previously,
+ some were influenced by error messages. When those error messages contained
+ '%p' (amongst other possibilities), String.format() would throw an exception.
 - [changed] Some SDK errors that represent common mistakes (such as permission
  denied or a missing index) will automatically be logged as a warning in
  addition to being surfaced via the API.