doc: change package index to default branch main

Author: fpistm

Diff for: README.md

@@ -34,9 +34,11 @@ This repo is available as a package usable with [Arduino Boards Manager](https:/
 
 Add 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 2.0.0 this link has changed.
+**Warning**: 
+* Default branch has changed to *main*.
+* Since core release 2.0.0 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.