Add section about C++17 attributes (#50)

* Add section about C++17 attributes

Explains, in a single piece of code, about [[nodiscard]], [[fallthrough]] and [[maybe_unused]]

CPP17.md

@@ -17,6 +17,7 @@ C++17 includes the following new language features:
 - [constexpr if](#constexpr-if)
 - [utf-8 character literals](#utf-8-character-literals)
 - [direct-list-initialization of enums](#direct-list-initialization-of-enums)
+- [New standard attributes](#new-standard-attributes)
 
 C++17 includes the following new library features:
 - [std::variant](#stdvariant)
@@ -237,6 +238,25 @@ byte d = byte{1}; // OK
 byte e = byte{256}; // ERROR
 ```
 
+### New standard attributes
+C++17 introduces three new attributes: `[[fallthrough]]`, `[[nodiscard]]` and `[[maybe_unused]]`:
+```c++
+// Will warn if return of foo() is ignored
+[[nodiscard]] int foo();
+int main() {
+  int a {1};
+  switch (a) {
+      // Indicates that falling through on case 1 is intentional
+      case 1: [[fallthrough]]
+      case 2:
+      // Indicates that b might be unused, such as on production builds
+        [[maybe_unused]] int b = foo();
+        assert(b > 0);
+        break;
+  }
+}
+```
+
 ## C++17 Library Features
 
 ### std::variant