Added Ex. 38-43 for pointers, updated README

Added topics beyond the language basics from ziglearn.org
to the README. That's a lot of exercises. I'd like to keep
it under 100, though!

42_pointers4.zig

@@ -0,0 +1,33 @@
+//
+// Now let's use pointers to do something we haven't been
+// able to do before: pass a value by reference to a function!
+//
+const std = @import("std");
+
+pub fn main() void {
+    var num: u8 = 1;
+    var more_nums = [_]u8{ 1, 1, 1, 1 };
+
+    // Let's pass a reference to num to our function and print it:
+    makeFive(&num);
+    std.debug.print("num: {}, ", .{num});
+
+
+    // Now something interesting. Let's pass a reference to a
+    // specific array value:
+    makeFive(&more_nums[2]);
+
+    // And print the array:
+    std.debug.print("more_nums: ", .{});
+    for (more_nums) |n| {
+        std.debug.print("{} ", .{n});
+    }
+
+    std.debug.print("\n", .{});
+}
+
+// This function should take a reference to a u8 value and set it
+// to 5.
+fn makeFive(x: *u8) void {
+    ??? = 5; // fix me!
+}