Add @echarles to core

This adds Eric Charles to the core team page on the README following a discussion on https://github.com/jupyterlab/team-compass/issues/66.

Once this is merged we should add him to the proper Github team.

README.md

@@ -146,6 +146,7 @@ JupyterLab's current maintainers are listed in alphabetical order, with affiliat
 - Tim George, Cal Poly (UI/UX design, strategy, management, user needs analysis)
 - Brian Granger, AWS (co-creator, strategy, vision, management, UI/UX design,
   architecture).
+- Eric Charles, Datalayer, (general development, extensions)
 - Jason Grout, Bloomberg (co-creator, vision, general development).
 - Max Klein, JPMorgan Chase (UI Package, build system, general development, extensions).
 - Fernando Perez, UC Berkeley (co-creator, vision).