Readme: Build instructions without rustup

Author: dcz

README.md

@@ -56,6 +56,24 @@ To build all the provided examples run
 cargo build --examples --release --all-features
 ```
 
+#### Building everything including core
+
+An option for using only distribution packaging, no rustup, is to let the compiler pretend it's unstable and build the `core` crate.
+
+Install rust and its sources.
+
+Fedora:
+
+```
+sudo dnf install rust rust-src
+```
+
+Use the following command to build instead:
+
+```
+RUSTC_BOOTSTRAP=1 cargo build -Z build-std --examples --release --all-features
+```
+
 ### Using dfu-util for Flashing
 
 The GD32VF103 contains a [DFU](https://www.usb.org/sites/default/files/DFU_1.1.pdf)