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!

41_pointers3.zig

@@ -0,0 +1,41 @@
+//
+// The tricky part is that the pointer's mutability (var vs const) refers
+// to the ability to change what the pointer POINTS TO, not the ability
+// to change the VALUE at that location!
+//
+//     const locked: u8 = 5;
+//     var unlocked: u8 = 10;
+//
+//     const p1: *const u8 = &locked;
+//     var   p2: *const u8 = &locked;
+//
+// Both p1 and p2 point to constant values which cannot change. However,
+// p2 can be changed to point to something else and p1 cannot!
+//
+//     const p3: *u8 = &unlocked;
+//     var   p4: *u8 = &unlocked;
+//     const p5: *const u8 = &unlocked;
+//     var   p6: *const u8 = &unlocked;
+//      
+// Here p3 and p4 can both be used to change the value they point to but
+// p3 cannot point at anything else.
+// What's interesting is that p5 and p6 act like p1 and p2, but point to
+// the value at "unlocked". This is what we mean when we say that we can
+// make a constant reference to any value!
+//
+const std = @import("std");
+
+pub fn main() void {
+    var foo: u8 = 5;
+    var bar: u8 = 10;
+
+    // Please define pointer "p" so that it can point to EITHER foo or
+    // bar AND change the value it points to!
+    ??? p: ??? = undefined;
+    
+    p = &foo;
+    p.* += 1;
+    p = &bar;
+    p.* += 1;
+    std.debug.print("foo={}, bar={}\n", .{foo, bar});
+}