Update repositories.txt

[Arduino15]

Added the A15RGB library for easy RGB LED control.

[github-actions]

Arduino Lint has suggestions for possible improvements to https://github.com/Arduino15/A15RGB:

Linting library in A15RGB
WARNING: library.properties maintainer value Arduino15 <[email protected]> starts with "Arduino". 3rd party libraries  
         are not maintained by Arduino.                                                                                 
         See: https://arduino.github.io/arduino-cli/latest/library-specification/#libraryproperties-file-format         
         (Rule LP027)                                                                                                   

Linter results for project: 0 ERRORS, 1 WARNINGS

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Linting sketch in A15RGB/examples/HSV

Linter results for project: no errors or warnings

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Linting sketch in A15RGB/examples/blinking

Linter results for project: no errors or warnings

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Linting sketch in A15RGB/examples/gradient

Linter results for project: no errors or warnings

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Linting sketch in A15RGB/examples/palette

Linter results for project: no errors or warnings

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Linting sketch in A15RGB/examples/test

Linter results for project: no errors or warnings

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Linting sketch in A15RGB/examples/testColor

Linter results for project: no errors or warnings

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Linter results for projects: 0 ERRORS, 1 WARNINGS

[github-actions]

Your submission has now been accepted! Thanks for your contribution to the Arduino Library Manager index.

The library(s) will be available for installation via Library Manager within a day's time.

You can check the logs from the Library Manager indexer for your library(s) here:
http://downloads.arduino.cc/libraries/logs/github.com/Arduino15/A15RGB/