Suggest installing graphviz before book building (#4565)

polkadot/roadmap/implementers-guide/README.md

@@ -3,6 +3,14 @@
 The implementers' guide is compiled from several source files with [`mdBook`](https://github.com/rust-lang/mdBook).
 To view it live, locally, from the repo root:
 
+Ensure graphviz is installed:
+```sh
+brew install graphviz # for macOS
+sudo apt-get install graphviz # for Ubuntu/Debian
+```
+
+Then install and build the book:
+
 ```sh
 cargo install mdbook mdbook-linkcheck mdbook-graphviz
 mdbook serve roadmap/implementers-guide