Document how to add custom component library in a project

[alexbudarov]

see forum answer:
https://www.cuba-platform.com/discuss/t/masquerade-appmenu-openitem/10518/4?u=alexbudarov

You should register you custom config using Java ServiceLoader rules: https://docs.oracle.com/javase/9/docs/api/java/util/ServiceLoader.html

Deploying service providers on the class path

A service provider that is packaged as a JAR file for the class path is identified by placing a provider-configuration file in the resource directory META-INF/services . The name of the provider-configuration file is the fully qualified binary name of the service. The provider-configuration file contains a list of fully qualified binary names of service providers, one per line.

For example, suppose the service provider com.example.impl.StandardCodecs is packaged in a JAR file for the class path. The JAR file will contain a provider-configuration file named:

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META-INF/services/com.example.CodecFactory
Here you should use filename equal to FQN of ComponentConfig class.

that contains the line:

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com.example.impl.StandardCodecs # Standard codecs
And here you need to use FQN of your custom config.