Add readmes for all modules

sop-java/README.md

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+# SOP-Java
+
+Stateless OpenPGP Protocol for Java.
+
+This module contains interfaces that model the API described by the 
+[Stateless OpenPGP Command Line Interface](https://datatracker.ietf.org/doc/draft-dkg-openpgp-stateless-cli/) specification.
+
+This module is not a command line application! For that, see `sop-java-picocli`.
+
+## Why should I use this?
+
+If you need to use OpenPGP functionality like encrypting/decrypting messages, or creating/verifying
+signatures inside your application, you probably don't want to start from scratch and instead reuse some library.
+
+Instead of locking yourselves in by depending hard on that one library, you can simply depend on the interfaces from
+`sop-java` and plug in a library (such as `pgpainless-sop`) that implements said interfaces.
+
+That way you don't make yourself dependent from a single OpenPGP library and stay flexible.
+Should another library emerge, that better suits your needs (and implements `sop-java`), you can easily switch
+by swapping out the dependency with minimal changes to your code.
+
+## Why should I *implement* this?
+
+Did you create an [OpenPGP](https://datatracker.ietf.org/doc/html/rfc4880) implementation that can be used in the Java ecosystem?
+By implementing the `sop-java` interface, you can turn your library into a command line interface (see `sop-java-picocli`).
+This allows you to plug your library into the [OpenPGP interoperability test suite](https://tests.sequoia-pgp.org/)
+of the [Sequoia-PGP](https://sequoia-pgp.org/) project.