mention INSTALLING_FROM_SOURCE in README

Author: henrispriet

README.md

@@ -29,6 +29,10 @@ Read ["Installation"] from [The Book].
 ["Installation"]: https://doc.rust-lang.org/book/ch01-01-installation.html
 [The Book]: https://doc.rust-lang.org/book/index.html
 
+## Installing from source
+
+If you really want to install from source (though this is not recommended), see [INSTALLING_FROM_SOURCE.md](INSTALLING_FROM_SOURCE.md).
+
 ## Getting Help
 
 See https://www.rust-lang.org/community for a list of chat platforms and forums.