rust : add variables info.

dev/lang/lua/TODO.md

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+# Write about Lua.

dev/lang/rust/function.md

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+# Functions.
+
+# Declare.
+
+Declare functions using `fn` keyword.
+
+```rust
+fn main() {}
+
+fn main(x: i32) {}
+```

dev/lang/rust/todo.md

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-TODO: add rust wiki.

dev/lang/rust/variable.md

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+# Variables.
+
+# Definition.
+
+## Immutable.
+
+Use the `let` keyword to start declaration. Types are specified after the variable name.
+
+```rust
+let x: i32 = 10;
+```
+
+Type definition is optional, so you can drop it. I suggest specifying the type for global/class/etc variables, but dropping it in local scope like functions.
+
+```rust
+let x = "This is &str"; // Type is detected automatically: &str.
+```
+
+## Mutable.
+
+Add `mut` keyword after `let`.
+
+```rust
+let mut x: i32 = 10;
+x = 12;
+```
+
+## Constants.
+
+To declare constant, use the `const` keyword instead of `let`. Constant values always require type definition.
+
+```rust
+const NUMBER: i32 = 10;
+```
+
+# Shadowing.
+
+You can redefine a variable with the same name but with a different type. This is useful if you cast something, but want to keep the same variable name. To do this, just declare the variable again.
+
+```rust
+let x: &str = "&str"; // &str.
+let x: i32  = 10;     // i32.
+```