Cross-reference Building CircuitPython and debugging on SAMD21 guides in README.md

[dhalbert]

Put links to these in the toplevel README.md

https://learn.adafruit.com/building-circuitpython
https://learn.adafruit.com/debugging-the-samd21-with-gdb

[sommersoft]

And once it finally gets published? maybe? 😄

https://learn.adafruit.com/circuitpython-samd-debugging-w-atmel-studio-7

[ladyada]

@sommersoft is that guide ready for moderation?

[sommersoft]

not quite yet. still waiting on @brentru to finish his review (not calling you out! take your time), as well as another Discord user and @kattni (super-busy, as she is).

[ladyada]

ok thx!