[README] "Getting Started" section: Mention where the "Additional Boards Managers URLs" field is

[Michal-Martinek]

Mention that the "Additional Boards Managers URLs" field, in which you're expected to copy a URL, is in
File / Preferences

Also it would be nice to mention what to do next:

• goto boards manager
• select you new library
• install

This could have saved me few minutes.

[fpistm]

If you read carefully before that you have a link to the Arduino Board manager:
https://docs.arduino.cc/learn/starting-guide/cores/
It describes "How to install a third party core".

[fpistm]

Looking the README, it is explicitly wrote:

For full instructions on using the "Boards Manager", see the Getting Started page.

Then in the Getting Started page of the wiki:
You have all step to add the url then install the third party core:
https://github.com/stm32duino/Arduino_Core_STM32/wiki/Getting-Started#add-stm32-boards-support-to-arduino
including all step you required to be added.
So I close this issue, it is not valid as all step properly described.