Merge pull request #179 from zeenix/patch-1

Document that most enums representations require `std` or `alloc` features

Author: dtolnay

_src/enum-representations.md

@@ -40,6 +40,8 @@ In JSON and other self-describing formats, the externally tagged representation
 is often not ideal for readability. Serde provides attributes to select three
 other possible representations.
 
+**Note**: This is the only enums representation that requires neither `std` nor `alloc` Cargo features to be enabled.
+
 ## Internally tagged
 
 ```rust