Add link to Jelly to about.md

[Ostrzyciel]

GitHub issue resolved: N/A

Briefly describe the changes proposed in this PR:

Added a link to the Jelly serialization format implementation for RDF4J: https://w3id.org/jelly/jelly-jvm

Jelly is a binary RDF format with a strong performance focus. The implementation for RDF4J can be either used as a Maven dependency, or as a JAR plugin that you drop into your classpath.

Jelly was also implemented for Apache Jena. An implementation for rdflib is planned.

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PR Author Checklist (see the contributor guidelines for more details):

• [x] my pull request is self-contained
• [x] I've added tests for the changes I made (not applicable)
• [x] I've applied code formatting (you can use mvn process-resources to format from the command line)
• [x] I've squashed my commits where necessary
• [ ] every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change (do I need to this?)