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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• I've added tests for the changes I made (not applicable)
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