Mention that SERD is mandatory.

README.md

@@ -3,9 +3,9 @@ HDT keeps big RDF datasets compressed while maintaining efficient search and bro
 
 To compile the library run `make` under the directory `hdt-lib`, this will generate the library and tools.
 
-The implementation has the following optional dependencies:
+The implementation has the following dependencies:
+- [Serd](http://drobilla.net/software/serd/) This enables importing RDF data in the Turtle and N-Triples serialization formats specifically. The dependency is activated by default; to deactivate it, comment out the line `SERD_SUPPORT=true` in the `Makefile`.
 - [Raptor RDF Parser Library 2.x](http://librdf.org/raptor/) (optional) This enables importing RDF data in many serialization formats, e.g., RDF/XML, Turtle, N3, etc. The dependency is activated by default; to deactivate it, comment out the line `RAPTOR_SUPPORT=true` in the `Makefile`. If Raptor or Serd is not used, the library will only be able to load RDF data in N-Triples format.
-- [Serd](http://drobilla.net/software/serd/) (optional) This enables importing RDF data in the Turtle and N-Triples serialization formats specifically. The dependency is activated by default; to deactivate it, comment out the line `SERD_SUPPORT=true` in the `Makefile`. If Raptor or Serd is not used, the library will only be able to load RDF data in N-Triples format.
 - [libz](http://www.zlib.net/) (optional) Enables loading N-Triples files compressed with GZIP (e.g., `file.nt.gz`) and gzipped HDTs (`file.hdt.gz`). The dependency is activated by default; to deactivate it, comment out the line `LIBZ_SUPPORT=true` in the `Makefile`.
 - [Kyoto Cabinet](http://fallabs.com/kyotocabinet/) (optional) Enables generating big RDF datasets on machines without much RAM memory, by creating a temporary Kyoto Cabinet database. The dependency is deactivated by default; to activate it, uncomment the line `KYOTO_SUPPORT=true` in the `Makefile` and edit the library include path (`INCLUDES=`) as needed.