Update README.md (flameshot-org#4151)

* Update README.md

Updated instructions to open the app first time on latest mac os
source:
1. flameshot-org#3572
2. https://flameshot.org/docs/guide/troubleshooting/#:~:text=%F0%9F%94%97%20%E2%80%9Cflameshot%E2%80%9D%20app%20cannot%20be%20opened&text=Open%20System%20Settings%20%2D%3E%20Privacy%20%26,not%20from%20an%20identified%20developer%22.

* Update README.md

Updated instructions to go to settings -< privacy and security also as an option

* Update README.md

remove extra new lines

Author: renjith100

README.md

@@ -377,6 +377,10 @@ follow the steps below:
 1. Right-Click on "flameshot.app" and choose "Open" from the context menu
 2. In the dialog click "Open"
 
+On MacOs 15 and above, you will have to go to system settings -> privacy and security after doing this and click "Open Anyway" or you can open flameshot first time with the following command.
+
+```sudo xattr -rd com.apple.quarantine /Applications/flameshot.app```
+
 After following all those steps above, `flameshot` will open without problems in your Mac.
 
 ### Windows