Available since 1.4 (source)
    Map from reader tag symbols to data reader Vars.
     When Clojure starts, it searches for files named 'data_readers.clj'
    and 'data_readers.cljc' at the root of the classpath. Each such file
    must contain a literal map of symbols, like this:
         foo/baz my.project/baz}
     The first symbol in each pair is a tag that will be recognized by
    the Clojure reader. The second symbol in the pair is the
    fully-qualified name of a Var which will be invoked by the reader to
    parse the form following the tag. For example, given the
    data_readers.clj file above, the Clojure reader would parse this
         #foo/bar [1 2 3]
     by invoking the Var #' on the vector [1 2 3]. The
    data reader function is invoked on the form AFTER it has been read
    as a normal Clojure data structure by the reader.
     Reader tags without namespace qualifiers are reserved for
    Clojure. Default reader tags are defined in
    clojure.core/default-data-readers but may be overridden in
    data_readers.clj, data_readers.cljc, or by rebinding this Var.