Add Rhai book.

doc/src/language/constants.md

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+Constants
+=========
+
+{{#include ../links.md}}
+
+Constants can be defined using the `const` keyword and are immutable.
+
+Constants follow the same naming rules as [variables].
+
+```rust
+const x = 42;
+print(x * 2);       // prints 84
+x = 123;            // <- syntax error: cannot assign to constant
+```
+
+Constants must be assigned a _value_, not an expression.
+
+```rust
+const x = 40 + 2;   // <- syntax error: cannot assign expression to constant
+```