Add readmes for all modules

pgpainless-cli/README.md

@@ -1,15 +1,24 @@
-# PGPainless-SOP
+# PGPainless-CLI
 
-PGPainless-SOP is an implementation of the [Stateless OpenPGP Command Line Interface](https://tools.ietf.org/html/draft-dkg-openpgp-stateless-cli-01) specification based on PGPainless.
+PGPainless-CLI is an implementation of the [Stateless OpenPGP Command Line Interface](https://tools.ietf.org/html/draft-dkg-openpgp-stateless-cli-01) specification based on PGPainless.
+
+It plugs `pgpainless-sop` into `sop-java-picocli`.
 
 ## Build
 To build an executable, `gradle jar` should be sufficient. The resulting jar file can be found in `pgpainless-sop/build/libs/`.
 
 ## Execute
 
-Alternatively you can use the provided `./pgpainless-cli` script to execute PGPainless' Stateless Command Line Interface from within Gradle.
+The jar file produced in the step above is executable as is.
+
+```
+java -jar pgpainless-cli-XXX.jar help
+```
+
+Alternatively you can use the provided `./pgpainless-cli` script to directly build and execute PGPainless' Stateless Command Line Interface from within Gradle.
 
 To discover all available commands use
+
 ```
 ./pgpainless-cli help
 ```

pgpainless-core/README.md

@@ -0,0 +1,3 @@
+# PGPainless-Core
+
+Wrapper around Bouncycastle's OpenPGP implementation.

pgpainless-sop/README.md

@@ -0,0 +1,6 @@
+# PGPainless-SOP
+
+Implementation of the Stateless OpenPGP Protocol using PGPainless.
+
+This module implements `sop-java` using `pgpainless-core`.
+If your code depends on `sop-java`, this module can be used as a realization of those interfaces.

sop-java-picocli/README.md

@@ -0,0 +1,29 @@
+# SOP-Java-Picocli
+
+Implementation of the [Stateless OpenPGP Command Line Interface](https://tools.ietf.org/html/draft-dkg-openpgp-stateless-cli-01) specification.
+This terminal application allows generation of OpenPGP keys, extraction of public key certificates,
+armoring and de-armoring of data, as well as - of course - encryption/decryption of messages and creation/verification of signatures.
+
+## Install a SOP backend
+
+This module comes without a SOP backend, so in order to function you need to extend it with an implementation of the interfaces defined in `sop-java`.
+An implementation using PGPainless can be found in the module `pgpainless-sop`, but it is of course possible to provide your
+own implementation.
+
+Just install your SOP backend by calling 
+```java
+// static method call prior to execution of the main method
+SopCLI.setSopInstance(yourSopImpl);
+```
+
+## Usage
+
+To get an overview of available commands of the application, execute
+```shell
+java -jar sop-java-picocli-XXX.jar help
+```
+
+If you just want to get started encrypting messages, see the module `pgpainless-cli` which initializes
+`sop-java-picocli` with `pgpainless-sop`, so you can get started right away without the need to manually wire stuff up.
+
+Enjoy!

sop-java/README.md

@@ -0,0 +1,27 @@
+# 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.