Add loopj/open-rts to library manager repositories list

[loopj]

No description provided.

[github-actions]

Arduino Lint has suggestions for possible improvements to https://github.com/loopj/open-rts:

Linting library in open-rts
WARNING: Git submodule detected. Library Manager installations and installations from GitHub's "Download ZIP" will only 
         contain an empty folder in place of the submodule. (Rule LS004)                                                
INFO: No header file found matching library name (Open_RTS.h). Best practices are for primary header filename to match  
      library name.                                                                                                     
      See: https://arduino.github.io/arduino-cli/latest/library-specification/#libraryproperties-file-format            
      (Rule LS008)                                                                                                      
WARNING: library.properties name Open RTS contains spaces. Although supported, best practices is to not use spaces.
         See: https://arduino.github.io/arduino-cli/latest/library-specification/#libraryproperties-file-format    
         (Rule LP015)                                                                                              

Linter results for project: 0 ERRORS, 2 WARNINGS

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Linting sketch in open-rts/examples/arduino/oled-sniffer

Linter results for project: no errors or warnings

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Linting sketch in open-rts/examples/arduino/receiver

Linter results for project: no errors or warnings

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Linting sketch in open-rts/examples/arduino/remote

Linter results for project: no errors or warnings

-------------------

Linting sketch in open-rts/examples/arduino/sniffer

Linter results for project: no errors or warnings

-------------------

Linter results for projects: 0 ERRORS, 2 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/loopj/open-rts/