Adding myself (Justo Karell) to the community map

## Blast Intro: Justo Karell to Community Map ##
Hi! I'm Justo, adjunct at Stevens Institute of Technology working on cut-cell finite-volume spectral stability and teaching deep learning/NLP. I use WarpX for ion engine simulations, xenon PIC-MCC plasma simulations. I'm interested in helping out with anyone's research as well.

**Email**: justokarell@gmail.com or jkarell@stevens.edu
**Location**: Stamford, CT / NYC, NY
**GitHub**: https://github.com/justokarell/
**Research interests**: PIC simulations, MHD stability, ion propulsion, Mathematical Fluid Dynamics, Optimization (of computational time and resources)
**LinkedIn** (in case anyone's interested): https://www.linkedin.com/in/justokarell/
**Website**: https://justokarell.today/

The issue supplies Justo Karell's profile details and links but names no file or test. Start by locating the community-map data format and existing contributor entries, then add the supplied information in the same format. Done means the profile appears on the map with the listed location, interests, contact links, and GitHub account.

Written by the indexing model from the issue text.