Update HISTORY.md

HISTORY.md

@@ -6,12 +6,14 @@
     * syntax tree format for translatable input options and constants: "[choice]"
 * **Notable Changes:**
 * **Notable Fixes:**
+    * strings library: substrings handle negative indices as documented, thanks, Brian!
 * **Documentation Updates:**
 * **Translation Updates:**
 
 ### 2022-01-05
 * new dev version
 * blocks: added syntax tree representation for translated input options and constants
+* strings library update, thanks, Brian!
 
 ## 7.0.3:
 * **New Features:**