Merge pull request #178 from rdfhdt/doc-old-serd

DOC: Added installation instruction when Serd version is too old.

Author: LaurensRietveld

README.md

@@ -65,6 +65,12 @@ 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