Add Rhai book.

doc/src/language/if.md

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+`if` Statement
+==============
+
+{{#include ../links.md}}
+
+```rust
+if foo(x) {
+    print("It's true!");
+} else if bar == baz {
+    print("It's true again!");
+} else if ... {
+        :
+} else if ... {
+        :
+} else {
+    print("It's finally false!");
+}
+```
+
+All branches of an `if` statement must be enclosed within braces '`{`' .. '`}`', even when there is only one statement.
+Like Rust, there is no ambiguity regarding which `if` clause a statement belongs to.
+
+```rust
+if (decision) print("I've decided!");
+//            ^ syntax error, expecting '{' in statement block
+```
+
+
+`if`-Expressions
+---------------
+
+Like Rust, `if` statements can also be used as _expressions_, replacing the `? :` conditional operators
+in other C-like languages.
+
+```rust
+// The following is equivalent to C: int x = 1 + (decision ? 42 : 123) / 2;
+let x = 1 + if decision { 42 } else { 123 } / 2;
+x == 22;
+
+let x = if decision { 42 }; // no else branch defaults to '()'
+x == ();
+```