add some comments about alloc in no_std

README.md

@@ -94,6 +94,14 @@ To do that add the following to your Cargo.toml:
 serde = { version = "1.0.27", default-features = false }
 ```
 
+Also, note that this implies that there is no memory allocator enabled by
+default. Therefore, if you have access to heap memory and want to use modules
+from the standard library such as String, Vec or similar you need to explicitly
+define which memory allocator to use. There are a number of different memory
+allocators available for that, for example
+[alloc](https://doc.rust-lang.org/nightly/alloc/index.html) that the standard
+library in Rust is using.
+
 ## License
 
 Serde is licensed under either of