Add a link to fortran90.org

I plan to migrate information from there to fortran-lang.org, but until
this happens, it would be good to at least provide the link, so that
people who are new to Fortran can use the site to learn modern Fortran.

Author: certik

_data/learning.yml

@@ -47,7 +47,11 @@ reference-links:
   - name: Fortran wiki
     url: http://fortranwiki.org/
     description: A rich collection of Fortran articles and resources in an editable wiki format
-    
+
+  - name: Fortran 90 org
+    url: https://www.fortran90.org/
+    description: Fortran Best Practices guide, Python/Fortran Rosetta Stone, Fortran FAQ
+
   - name: "Fortran 2018 Standard Interpretation Document"
     url: https://j3-fortran.org/doc/year/18/18-007r1.pdf
     description: "J3/18-007r1 F2018, specification of the base Fortran 2018 language"