Add README for sop module

pgpainless-sop/README.md

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+# PGPainless-SOP
+
+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.
+
+## Build
+To build an executable, `gradle jar` should be sufficient. The resulting jar file can be found in `pgpainless-sop/build/libs/`.
+
+## Execute
+You can now use the jar file like described in the stateless OpenPGP cli document.
+
+An example call may look like this:
+```
+java -jar pgpainless-sop-X.X.X.jar generate-key "Alice <alice@wonderland.lit>"
+```
+
+To discover all commands use
+```
+java -jar pgpainless-sop-X.X.X.jar help
+```
+
+Enjoy!