Update changelog

Author: cmaglie

build/shared/revisions.txt

@@ -1,7 +1,8 @@
-ARDUINO 1.8.15 Not yet release
+ARDUINO 1.8.16 2021.09.02
 
 [ide]
 * Removed the very outdated off-line documentation.
+* Do not crash if a malformed boards index URL is entered in preferences.
 
 [wifi-firmware]
 * Added latest firmwares (up to version 1.4.8) for NINA-based boards