Add no_std note

Author: febo

programs/system/README.md

@@ -4,6 +4,8 @@ This crate contains [`pinocchio`](https://crates.io/crates/pinocchio) helpers to
 
 Each instruction defines an `struct` with the accounts and parameters required. Once all values are set, you can call directly `invoke` or `invoke_signed` to perform the CPI.
 
+This is a `no_std` crate.
+
 > **Note:** The API defined in this crate is subject to change.
 
 ## Examples