Add Rhai book.

doc/src/engine/optimize/optimize-levels.md

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+Optimization Levels
+==================
+
+{{#include ../../links.md}}
+
+Set Optimization Level
+---------------------
+
+There are actually three levels of optimizations: `None`, `Simple` and `Full`.
+
+* `None` is obvious - no optimization on the AST is performed.
+
+* `Simple` (default) performs only relatively _safe_ optimizations without causing side-effects
+  (i.e. it only relies on static analysis and will not actually perform any function calls).
+
+* `Full` is _much_ more aggressive, _including_ running functions on constant arguments to determine their result.
+  One benefit to this is that many more optimization opportunities arise, especially with regards to comparison operators.
+
+An [`Engine`]'s optimization level is set via a call to `Engine::set_optimization_level`:
+
+```rust
+// Turn on aggressive optimizations
+engine.set_optimization_level(rhai::OptimizationLevel::Full);
+```