Update design/01-proc-macro.md

Co-authored-by: Marek Kaput <[email protected]>
Signed-off-by: maciektr <[email protected]>

design/01-proc-macro.md

@@ -90,6 +90,19 @@ version = "0.1.0"
 Automatically generated Cargo manifest file:
 
 ```toml
+# Code generated by scarb fetch; DO NOT EDIT.
+#
+# The Cargo.toml file for Cairo procedural macros crates is entirely managed by Scarb,
+# the Cairo package manager. Scarb regenerates this file whenever it processes Scarb.toml.
+# The simplest way to trigger this, is to run the `scarb fetch` command.
+#
+# If you want to provide custom data in this file, include it in Scarb.toml under
+# [tool.cargo.pkg] section. For example, to add Rust dependencies, type the following:
+#
+#     [tool.cargo.pkg.dependencies]
+#     serde = "1"
+#     serde_json = "1"
+
 [package]
 name = "ignore-plugin"
 version = "0.1.0"