Add build status badge to README

[Khalian]

Thanks for making a Pull Request!
Please fill out this template so that you can be sure that your PR does everything it needs to be accepted.

Description

What does your PR do?
Add a github CI CD badge for builds.

Is the PR checklist necessary for a README change?

PR Checklist

• Did you format your code using format.bash?
• Does the VST3 plugin pass all of the unit tests in the VST3PluginTestHost? (Download it as part of the VST3 SDK here.)
  • Windows
  • macOS
• Does your PR add, remove, or rename any plugin parameters? If yes...
  • Have you ensured that the plug-in unserializes correctly?
  • Have you ensured that older versions of the plug-in load correctly? (See Unserialization.cpp.)
• Does your PR add or remove any graphical assets? If yes, are they defined in config.h and added in the two required locations in main.rc?