Add Rhai book.

doc/src/start/builds/wasm.md

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+Building to WebAssembly (WASM)
+=============================
+
+{{#include ../../links.md}}
+
+It is possible to use Rhai when compiling to WebAssembly (WASM). This yields a scripting engine (and language)
+that can be run in a standard web browser. Why you would want to is another matter... as there is already
+a nice, fast, complete scripting language for the the common WASM environment (i.e. a browser) - and it is called JavaScript.
+But anyhow, do it because you _can_!
+
+When building for WASM, certain features will not be available, such as the script file API's and loading modules
+from external script files.
+
+Also look into [minimal builds] to reduce generated WASM size.  As of this version, a typical, full-featured
+Rhai scripting engine compiles to a single WASM file less than 200KB gzipped. When excluding features that are
+marginal in WASM environment, the gzipped payload can be further shrunk to 160KB.
+
+In benchmark tests, a WASM build runs scripts roughly 1.7-2.2x slower than a native optimized release build.