[NFC] add an entry describing changes introduced by swiftlang#2937

CHANGELOG.md

@@ -1,6 +1,12 @@
 Note: This is in reverse chronological order, so newer entries are added to the top.
 
 
+Swift 5.3.1
+-----------
+
+* [#2937](https://github.com/apple/swift-package-manager/pull/2937)
+  The Swift tools version specification at the top of each package manifest now accepts any combination of horizontal whitespace characters between `//` and `swift-tools-version`, if and only if the specified version > 5.3. For example, `//swift-tools-version:5.3.1` and `//	  swift-tools-version:5.4` are valid.
+
 Swift 4.2
 ---------