Add infrastructure folder

and diagram. This complements #118

Author: pllim

infrastructure/README.md

@@ -0,0 +1,7 @@
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.4390109.svg)](https://doi.org/10.5281/zenodo.4390109)
+
+The file `astropy_infrastructure.png` represents roughtly the state of
+Astropy infrastructure at the date it was mentioned on the diagram.
+It is also archived at the Zenodo DOI mentioned above.
+
+Related issue: https://github.com/astropy/astropy-project/issues/118