Add build documentation for Arch Linux

docs/development/Building in Arch Linux.md

@@ -0,0 +1,56 @@
+# Building in Arch Linux
+
+To build BetaFlight, first the ARM toolchain has to be installed:
+
+## Setup GNU ARM Toolchain
+
+```bash
+pacman -S arm-none-eabi-gcc arm-none-eabi-newlib arm-none-eabi-binutils
+```
+
+For running tests, the `libblocksruntime` package from the AUR also has to be installed:
+``` bash
+trizen -S libblocksruntime
+```
+
+At the time of writing, the repository version of GCC is 9.2.0, unlike 9.2.1 which BetaFlight expects.
+If you get the following error, you can override the GCC version in `make/locak.mk` before proceeding:
+```bash
+$ make
+make/tools.mk:301: *** **ERROR** your arm-none-eabi-gcc is '9.2.0', but '9.2.1' is expected. Override with 'GCC_REQUIRED_VERSION' in make/local.mk or run 'make arm_sdk_install' to install the right version automatically in the tools folder of this repo.  Stop.
+$ echo "GCC_REQUIRED_VERSION = 9.2.0" >> make/local.mk
+```
+
+## Building
+
+After the ARM toolchain is installed, you should be able to build from source.
+```bash
+cd src
+git clone git@github.com:betaflight/betaflight.git
+cd betaflight
+make MATEKF411
+```
+
+You'll see a set of files being compiled, and finally linked, yielding both an ELF and then a HEX:
+```
+...
+$ ls -l obj/
+total 1516
+-rwxr-xr-x 1 s-ol s-ol  428584 Jan 11 12:04 betaflight_4.2.0_MATEKF411.bin*
+-rw-r--r-- 1 s-ol s-ol 1136991 Jan 11 12:04 betaflight_4.2.0_MATEKF411.hex
+drwxr-xr-x 3 s-ol s-ol    4096 Jan 11 12:04 main/
+```
+
+You can use the Betaflight-Configurator to flash the `obj/betaflight_4.2.0_MATEKF411.hex` file.
+
+## Updating and rebuilding
+
+Navigate to the local betaflight repository and use the following steps to pull the latest changes and rebuild your version of betaflight:
+
+```bash
+cd src/betaflight
+git reset --hard
+git pull
+make clean
+make
+```