doc: explain SPARQL endpoint

doc/endpoints.md

+Endpoints
+=========
+
+With ULO/RDF triplets imported into a database, in our case GraphDB, we
+have all data available for querying. There are multiple approaches
+to querying such triplet stores.
+
+SPARQL
+------
+
+SPARQL [1] is a standardized query language for RDF triplet data. The spec
+includes not just syntax and semantics of the language itself, but also
+a standardized REST interface for querying databases.
+
+Various implementations of this standard, e.g. [2], are available so
+using SPARQL has the advantage of making us independent of a specific
+programming language or environment.
+
+SPARQL is inspired by SQL, a simple query that returns all triplets
+in the store looks like
+
+	SELECT *
+	WHERE { ?s ?p ?o }
+
+where `s`, `p` and `o` are query variables. The result of a query are
+valid substitutions for the query variables. In this case, the
+database would return a table of all triplets in the store sorted by
+subject `s`, predicate `p` and object `o`.
+
+Of course, queries might return a lot of data. Importing just the
+Isabelle exports [3] into GraphDB results in >200M triplets. This
+is solved with pagination techniques [4].
+
+References
+----------
+
+[1] https://www.w3.org/TR/rdf-sparql-query/
+
+[2] https://godoc.org/github.com/knakk/sparql
+
+[3] https://gl.mathhub.info/Isabelle
+
+[4] https://stackoverflow.com/questions/27488403/paginating-sparql-results