Add a BUILDING.md - #2370

BUILDING.md

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+# Building CircuitPython
+
+Welcome to CircuitPython!  
+
+This document is a quick-start guide only.
+
+Detailed guides on how to build CircuitPython can be found in the Adafruit Learn system at
+https://learn.adafruit.com/building-circuitpython/.
+
+## Setup
+
+Please ensure you setup your build environment appropriately, as per the guide.  You will need:
+
+* Linux: https://learn.adafruit.com/building-circuitpython/linux
+* MacOS: https://learn.adafruit.com/building-circuitpython/macos
+* Windows Subsystem for Linux (WSL): https://learn.adafruit.com/building-circuitpython/windows-subsystem-for-linux
+
+### Submodules
+
+This project has a bunch of git submodules.  You will need to update them regularly.
+
+    git submodule sync
+    git submodule update --init --recursive
+
+### mpy-cross
+
+As part of the build process, mpy-cross is needed to compile .py files into .mpy files.
+To compile (or recompile) mpy-cross:
+
+    make -C mpy-cross 
+
+# Building
+
+There a number of ports of CircuitPython!  To build for your board, change to the appropriate ports directory and build.
+
+Examples:
+
+    cd ports/atmel-samd
+    make BOARD=circuitplayground_express
+
+    cd ports/nrf
+    make BOARD=circuitplayground_bluefruit
+
+If you aren't sure what boards exist, have a peek in the boards subdirectory of your port.
+
+Useful make arguments or targets:
+
+    make BOARD=circuitplayground_express clean
+
+# Testing
+
+If you are working on changes to the core language, you might find it useful to run the test suite.
+The test suite in the top level `tests` directory.  It needs the unix port to run.
+
+    cd ports/unix
+    make axtls
+    make micropython
+
+Then you can run the test suite:
+
+    cd ../../tests
+    ./run-tests
+
+A successful run will say something like
+
+    676 tests performed (19129 individual testcases)
+    676 tests passed
+    30 tests skipped: buffered_writer builtin_help builtin_range_binop class_delattr_setattr cmd_parsetree extra_coverage framebuf1 framebuf16 framebuf2 framebuf4 framebuf8 framebuf_subclass mpy_invalid namedtuple_asdict non_compliant resource_stream schedule sys_getsizeof urandom_extra ure_groups ure_span ure_sub ure_sub_unmatched vfs_basic vfs_fat_fileio1 vfs_fat_fileio2 vfs_fat_more vfs_fat_oldproto vfs_fat_ramdisk vfs_userfs
+
+# Debugging
+
+The easiest way to debug CircuitPython on hardware is with a JLink device, JLinkGDBServer, and `gcc-arm-embedded`.   
+Instructions can be found at https://learn.adafruit.com/debugging-the-samd21-with-gdb
+
+If using JLink, you'll need both the `JLinkGDBServer` and `arm-non-eabi-gdb-py` running.
+
+Example:
+
+    JLinkGDBServer -if SWD -device ATSAMD51J19
+    arm-none-eabi-gdb-py build-metro_m4_express/firmware.elf -iex "target extended-remote :2331"
+ 
+