hugo release-number 21.0, + link -> PhysRef

hugo/content/_index.md

@@ -21,6 +21,7 @@ Some features of the software are not yet documented.
 Complementary information can be found in the
 
 * Reference Paper: [J. Appl. Cryst. 2020](http://journals.iucr.org/j/issues/2020/01/00/ge5067/ge5067.pdf)
+* [Physics Reference]({{% url-releases %}}/BornAgain-PhysicsReference.pdf)
 
 Older course materials are at
 

hugo/hugo.toml

@@ -19,7 +19,7 @@ PygmentsStyle = "vs"
   email = "contact@bornagainproject.org"
   url_home = "https://bornagainproject.org"
 
-  release_number = "20.2"
+  release_number = "21.0"
   # version name is either major_release_number or "git-main" (must agree with baseURL)
   version_name = "git-main"