Readme: Build instructions without rustup

Author: dcz

.cargo/config.toml

@@ -1,4 +1,5 @@
 [target.riscv32imac-unknown-none-elf]
+linker='lld'
 runner = 'riscv64-unknown-elf-gdb -x openocd.gdb'
 rustflags = [
   "-C", "link-arg=-Tmemory-c8.x",

README.md

@@ -56,6 +56,31 @@ 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, and the lld linker.
+
+Fedora:
+
+```
+sudo dnf install rust rust-src lld
+```
+
+In `.cargo/config` add:
+
+```
+[target.riscv32imac-unknown-none-elf]
+linker = 'lld'
+```
+
+Use the following command to build:
+
+```
+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)