Updated list of publications using GemPy

[flohorovicic]

Description

Updated list of publications using GemPy on README

Checklist

No code contribution :-)

• My code follows the PEP 8 style guidelines.
• My code uses type hinting for function and method arguments and return values.
• My code contains descriptive and helpful docstrings
  which are formatted per the Google Python Style Guidelines.
• I have created tests which entirely cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests)
  or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New and existing tests pass locally with my changes.