add section about associative arrays

hands-on_dlang.md

@@ -721,3 +721,17 @@ Slices are the most prominent example of `RandomAccessRange`s in
 The D standard library provides a huge arsenal of lazy range algorithm
 functions. Most of them can be found in in the `std.range` and `std.algorithm`
 packages.
+
+### Associative arrays
+
+D has builtin hashmaps, which are called _associative arrays_:
+
+```D
+int[string] map; // keys of type string, values of type int
+map["key1"] = 10; // insertion or modification, if the key already exists
+if ("key1" in map) { // checking if a key is in an associative array
+    writeln("key1 is in map");
+}
+assert(map.length == 1); // associative arrays provide a .length property
+map.remove("key1"); // remove a key from an associative array
+```