Mention rustfmt requirement in the README (#2977)

Author: leoyvens

README.md

@@ -16,6 +16,7 @@ For detailed instructions and more context, check out the [Getting Started Guide
 To build and run this project you need to have the following installed on your system:
 
 - Rust (latest stable) – [How to install Rust](https://www.rust-lang.org/en-US/install.html)
+  - Note that `rustfmt`, which is part of the default Rust installation, is a build-time requirement.
 - PostgreSQL – [PostgreSQL Downloads](https://www.postgresql.org/download/)
 - IPFS – [Installing IPFS](https://docs.ipfs.io/install/)