Add Rhai book.

doc/src/language/while.md

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+`while` Loop
+============
+
+{{#include ../links.md}}
+
+```rust
+let x = 10;
+
+while x > 0 {
+    x = x - 1;
+    if x < 6 { continue; }  // skip to the next iteration
+    print(x);
+    if x == 5 { break; }    // break out of while loop
+}
+```