Add download links for latest snapshot build from GitLab CI

CHANGELOG.md

 # Changelog
 
-## [(next version)](./README.md#for-developers)
+## [(next version)](https://gitlab.cs.uni-duesseldorf.de/api/v4/projects/848/jobs/artifacts/master/raw/build/libs/prob2-jupyter-kernel-1.2.1-SNAPSHOT-all.jar?job=test)
 
 * Updated ProB 2 to version 4.0.0-SNAPSHOT.
 * Added a `:language` command to allow changing the language used to parse user input. For example `:language event_b` can be used to switch to Event-B syntax when a non-Event-B machine is loaded (or no machine at all).

README.md

@@ -10,6 +10,7 @@ This is a [Jupyter](https://jupyter.org/) kernel for the [ProB animator and mode
 
 * **[Download the latest version here](https://www3.hhu.de/stups/downloads/prob2-jupyter/prob2-jupyter-kernel-1.2.0-all.jar).**
 * Download links for previous versions can be found in the [changelog].
+* A [snapshot build](https://gitlab.cs.uni-duesseldorf.de/api/v4/projects/848/jobs/artifacts/master/raw/build/libs/prob2-jupyter-kernel-1.2.1-SNAPSHOT-all.jar?job=test) of the latest development version is also available. **Warning:** this is an unstable version that can contain bugs or breaking changes.
 
 See the [requirements](#requirements) and [installation instructions](#installation) for information on how to install and use the downloaded jar file.