Moved Old Serd mention to the Serd dep

Author: mielvds

README.md

@@ -46,6 +46,10 @@ dependencies installed:
     (e.g., Ubuntu)
   - `sudo dnf install serd serd-devel` on Red Hat-based distros (e.g.,
     Fedora)
+    
+  Sometimes the version of Serd that is distributed by package managers
+  is too old.  In that case, Serd can also be built manually: see
+  https://github.com/drobilla/serd for the installation instructions.
 
 ### Installation
 
@@ -65,12 +69,6 @@ Sometimes, the above instructions do not result in a working HDT
 installation.  This section enumerates common issues and their
 workaround.
 
-#### My version of Serd is too old
-
-Sometimes the version of Serd that is distributed by package managers
-is too old.  In that case, Serd can also be built manually: see
-https://github.com/drobilla/serd for the installation instructions.
-
 #### `./configure` cannot find Serd
 
 While running `./configure` you get a message similar to the