Add Rhai book.

doc/src/language/logic.md

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+Logic Operators
+==============
+
+{{#include ../links.md}}
+
+Comparison Operators
+-------------------
+
+Comparing most values of the same data type work out-of-the-box for all [standard types] supported by the system.
+
+However, if using a [raw `Engine`] without loading any [packages], comparisons can only be made between a limited
+set of types (see [built-in operators]).
+
+```rust
+42 == 42;               // true
+42 > 42;                // false
+"hello" > "foo";        // true
+"42" == 42;             // false
+```
+
+Comparing two values of _different_ data types, or of unknown data types, always results in `false`,
+except for '`!=`' (not equals) which results in `true`. This is in line with intuition.
+
+```rust
+42 == 42.0;             // false - i64 cannot be compared with f64
+42 != 42.0;             // true - i64 cannot be compared with f64
+
+42 > "42";              // false - i64 cannot be compared with string
+42 <= "42";             // false - i64 cannot be compared with string
+
+let ts = new_ts();      // custom type
+ts == 42;               // false - types cannot be compared
+ts != 42;               // true - types cannot be compared
+```
+
+Boolean operators
+-----------------
+
+| Operator | Description                           |
+| -------- | ------------------------------------- |
+| `!`      | Boolean _Not_                         |
+| `&&`     | Boolean _And_ (short-circuits)        |
+| `\|\|`   | Boolean _Or_ (short-circuits)         |
+| `&`      | Boolean _And_ (doesn't short-circuit) |
+| `\|`     | Boolean _Or_ (doesn't short-circuit)  |
+
+Double boolean operators `&&` and `||` _short-circuit_, meaning that the second operand will not be evaluated
+if the first one already proves the condition wrong.
+
+Single boolean operators `&` and `|` always evaluate both operands.
+
+```rust
+this() || that();       // that() is not evaluated if this() is true
+this() && that();       // that() is not evaluated if this() is false
+
+this() | that();        // both this() and that() are evaluated
+this() & that();        // both this() and that() are evaluated
+```
+
+Compound Assignment Operators
+----------------------------
+
+```rust
+let number = 5;
+number += 4;            // number = number + 4
+number -= 3;            // number = number - 3
+number *= 2;            // number = number * 2
+number /= 1;            // number = number / 1
+number %= 3;            // number = number % 3
+number <<= 2;           // number = number << 2
+number >>= 1;           // number = number >> 1
+```
+
+The `+=` operator can also be used to build [strings]:
+
+```rust
+let my_str = "abc";
+my_str += "ABC";
+my_str += 12345;
+
+my_str == "abcABC12345"
+```