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Community plugins are compiled and distributed with Source, however they are not actively maintained by the development team. Community member members are welcome to add plugins to this list and share them with other community members.
The core team does ensure that with each new release the plugins are re-compiled against each released version so that process doesn't need to be done manually each time there is a new release.
To add in you own your plugin into the plugins that are compiled and distributed you need access to the source code. Please follow the instruction in Accessing the source code.
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