Add Foyer graphic to README.md

[CalCraven]

PR Summary:

Add Foyer version of Mosdef Graphic to Readme page similar to what was done in mBuild PR#918

PR Checklist

---

• [x] Includes appropriate unit test(s)
• [x] Appropriate docstring(s) are added/updated
• [x] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Merging #443 (0ea575b) into master (c8e789b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #443   +/-   ##
=======================================
  Coverage   73.84%   73.84%           
=======================================
  Files          17       17           
  Lines        1839     1839           
=======================================
  Hits         1358     1358           
  Misses        481      481