doc: change package index to default branch main

fpistm · web-flow · commit 64888ab73049 · 2021-10-05T15:19:39.000+02:00
diff --git a/README.md b/README.md
@@ -25,9 +25,11 @@ This repo is available as a package usable with [Arduino Boards Manager](https:/
 
 Use this link in the "*Additional Boards Managers URLs*" field:
 
-https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json
+https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json
 
-**Warning**: Since core release 1.0.0 this link has changed.
+**Warning**:
+* Default branch has changed to main.
+* Since core release 1.0.0 this link has changed.
 
 For full instructions on using the "**Boards Manager**", see the [Getting Started](https://github.com/stm32duino/wiki/wiki/Getting-Started) page.
 
