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!

39_pointers.zig

@@ -0,0 +1,36 @@
+//
+// Check this out:
+//
+//     var foo: u8 = 5;      // foo is 5
+//     var bar: *u8 = &foo;  // bar is a pointer
+// 
+// What is a pointer? It's a reference to a value. In this example
+// bar is a reference to the memory space that current contains the
+// value 5.
+//
+// A cheatsheet given the above declarations:
+//
+//     u8         the type of a u8 value
+//     foo        the value 5
+//     *u8        the type of a pointer to a u8 value
+//     &foo       a reference to foo
+//     bar        a pointer to the value at foo
+//     bar.*      the value 5 (the dereferenced value "at" bar)
+//     
+// We'll see why pointers are useful in a moment. For now, see if you
+// can make this example work!
+//
+const std = @import("std");
+
+pub fn main() void {
+    var num1: u8 = 5;
+    var num1_pointer: *u8 = &num1;
+
+    var num2: u8 = undefined;
+
+    // Please make num2 equal 5 using num1_pointer!
+    // (See the "cheatsheet" above for ideas.)
+    num2 = ???;
+
+    std.debug.print("num1: {}, num2: {}\n", .{num1, num2});
+}