Add Rhai book.

doc/src/language/for.md

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+`for` Loop
+==========
+
+{{#include ../links.md}}
+
+Iterating through a range or an [array] is provided by the `for` ... `in` loop.
+
+```rust
+// Iterate through string, yielding characters
+let s = "hello, world!";
+
+for ch in s {
+    if ch > 'z' { continue; } // skip to the next iteration
+    print(ch);
+    if x == '@' { break; }   // break out of for loop
+}
+
+// Iterate through array
+let array = [1, 3, 5, 7, 9, 42];
+
+for x in array {
+    if x > 10 { continue; } // skip to the next iteration
+    print(x);
+    if x == 42 { break; }   // break out of for loop
+}
+
+// The 'range' function allows iterating from first to last-1
+for x in range(0, 50) {
+    if x > 10 { continue; } // skip to the next iteration
+    print(x);
+    if x == 42 { break; }   // break out of for loop
+}
+
+// The 'range' function also takes a step
+for x in range(0, 50, 3) {  // step by 3
+    if x > 10 { continue; } // skip to the next iteration
+    print(x);
+    if x == 42 { break; }   // break out of for loop
+}
+
+// Iterate through object map
+let map = #{a:1, b:3, c:5, d:7, e:9};
+
+// Property names are returned in random order
+for x in keys(map) {
+    if x > 10 { continue; } // skip to the next iteration
+    print(x);
+    if x == 42 { break; }   // break out of for loop
+}
+
+// Property values are returned in random order
+for val in values(map) {
+    print(val);
+}
+```