Add "Introduction to Fortran"

Introduction to Fortran
By: Sebastian Ehlert @awvwgk at @grimme-lab

https://qc2-teaching.readthedocs.io/en/latest/programming.html

https://github.com/grimme-lab/qc2-teaching/blob/main/doc/programming.rst

books/free-programming-books.md

@@ -1461,6 +1461,7 @@ Kerridge (PDF) (email address *requested*, not required)
 ### Fortran
 
 * [Fortran Wiki - Tutorials](http://fortranwiki.org/fortran/show/Tutorials) (HTML)
+* [Introduction to Fortran](https://qc2-teaching.readthedocs.io/en/latest/programming.html) - Sebastian Ehlert, Julius Stückrath, Marcel Mueller, Marcel Stahn (HTML)
 * [Modern Fortran in Science and Technology](https://modern-fortran-in-science-and-technology.readthedocs.io/en/latest) - Jonas Lindemann and Ola Dahlblom (HTML, [PDF](https://modern-fortran-in-science-and-technology.readthedocs.io/_/downloads/en/latest/pdf/), [EPUB](https://modern-fortran-in-science-and-technology.readthedocs.io/_/downloads/en/latest/epub/))
 * [Professional Programmer’s Guide to Fortran77 (2005)](https://www.star.le.ac.uk/~cgp/prof77.pdf) - Clive G. Page (PDF)
 * [Self Study Guide 2: Programming in Fortran 95](http://www.mrao.cam.ac.uk/~rachael/compphys/SelfStudyF95.pdf) - Dr Rachael Padman (PDF)