Add Rhai book.

doc/src/language/variables.md

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+Variables
+=========
+
+{{#include ../links.md}}
+
+Variables in Rhai follow normal C naming rules (i.e. must contain only ASCII letters, digits and underscores '`_`').
+
+Variable names must start with an ASCII letter or an underscore '`_`', must contain at least one ASCII letter,
+and must start with an ASCII letter before a digit.
+
+Therefore, names like '`_`', '`_42`', '`3a`' etc. are not legal variable names, but '`_c3po`' and '`r2d2`' are.
+Variable names are also case _sensitive_.
+
+Variables are defined using the `let` keyword. A variable defined within a statement block is _local_ to that block.
+
+```rust
+let x = 3;          // ok
+let _x = 42;        // ok
+let x_ = 42;        // also ok
+let _x_ = 42;       // still ok
+
+let _ = 123;        // <- syntax error: illegal variable name
+let _9 = 9;         // <- syntax error: illegal variable name
+
+let x = 42;         // variable is 'x', lower case
+let X = 123;        // variable is 'X', upper case
+x == 42;
+X == 123;
+
+{
+    let x = 999;    // local variable 'x' shadows the 'x' in parent block
+    x == 999;       // access to local 'x'
+}
+x == 42;            // the parent block's 'x' is not changed
+```