add section about CTFE

hands-on_dlang.md

@@ -1371,3 +1371,45 @@ assert(c == 5);
 immutable runTimeString = q{int c =5;};
 mixin(runTimeString); // compile error, runTimeString is not known at compile-time
 ```
+
+### Compile Time Function Evaluation (CTFE)
+
+In D, it is possible to execute functions at compile time. This is triggered on
+the call site, i.e. whenever a function is called in a context where the result
+needs to be known at compile time, the function will be evaluated at compile
+time. Some of those contexts are:
+* Template parameters
+* Initialization of `static` variables
+* Definition of `enum`s (i.e. compile time constants)
+* `static assert`, `static foreach` and `static if`
+* Template constraints
+
+There are some limitations to what a function executed at compile time may do.
+Essentially, the functions need to be *pure* in some sense, i.e. they may not
+have side effects such as IO.
+
+```D
+import std.stdio : writeln;
+
+auto sqrt(T)(T x) {
+    enum GoodEnough = 0.01;
+    import std.math : abs;
+    T z = x*x, old = 0;
+    int iter;
+    while (abs(z - old) > GoodEnough) {
+        old = z;
+        z -= ((z*z)-x) / (2*z);
+    }
+
+    return z;
+}
+
+void main() {
+    double n = 4.0;
+    writeln("The sqrt of runtime 4 = ",
+        sqrt(n));
+    static cn = sqrt(4.0); // calculated at compile time
+    writeln("The sqrt of compile time 4 = ",
+        cn);
+}
+```