Add some documentation about implemented extensions

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+# Extensions
+
+In addition to C11, several [GNU extensions] are implemented and more
+may be implemented in the future.
+
+## Implemented
+
+### [`__typeof__`]
+
+`__typeof__(E)` is a type specifier for the type of expression `E`. Arrays
+and function designator expressions do not decay into pointers, just
+like when used with `sizeof`.
+
+### Built-in functions and types
+
+- **`__builtin_alloca`**: Allocate memory on the stack.
+- **`__builtin_constant_p`**: Test whether the argument is a constant expression.
+- **`__builtin_inff`**: `float` positive infinity value.
+- **`__builtin_nanf`**: `float` quiet NaN value.
+- **[`__builtin_offsetof`]**: Return the offset of a member in a struct or union.
+- **`__builtin_types_compatible_p`**: Test whether the two types are compatible.
+- **`__builtin_va_arg`**: Built-in suitable for implementing the `va_arg` macro.
+- **`__builtin_va_copy`**: Built-in suitible for implementing the `va_copy` macro.
+- **`__builtin_va_end`**: Built-in suitible for implementing the `va_end` macro.
+- **`__builtin_va_list`**: Built-in suitable for implementing the `va_list` type.
+- **`__builtin_va_start`**: Built-in suitable for implementing the `va_start` macro.
+
+## Missing
+
+### [Statement expressions]
+
+In GNU C, you may use a compound statement as expressions when they are
+enclosed in parentheses. The last statement in the compound statement
+must be an expression statement, whose value is used as the result of
+the statement expression.
+
+### Empty declarations
+
+GNU C allows empty top-level declarations (i.e. `;`).
+
+### Empty initializer list
+
+GNU C allows empty initializer lists when initializing an object,
+for example
+
+```c
+struct {
+	int a, b;
+} s = {};
+```
+
+This behaves exactly the same as `{0}`.
+
+### Missing operand in conditional expression
+
+GNU C allows you to omit the second operand in conditional expressions,
+in which case the first operand is used. So `E1 ? : E2` behaves the same
+as `E1 ? E1 : E2`, except that `E1` is evaluated only once.
+
+[GNU extensions]: https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
+[`__typeof__`]: https://gcc.gnu.org/onlinedocs/gcc/Typeof.html
+[Statement expressions]: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html