Add documentation for Windows Defender making things slow

[julianneswinoga]

Testing exclusions for Windows Defender, full build, no .pio folder:

Test	Absolute Times	Build time
No exclusions	11:42:27,290 -> 11:49:58,025	450.7s
Exclude app folder	11:51:29,055 -> 11:58:52,675	443.6s
Exclude both app and .pio folder	12:00:24,547 -> 12:03:11,351	166.8s
Exclude *.py extension	12:04:45,371 -> 12:11:42,312	416.9s
Exclude platformio.exe process	12:13:11,923 -> 12:20:13,813	422.0s
Exclude pio.exe process	12:21:58,938 -> 12:29:12,484	433.5s

Maybe there could be an automatic addition to Windows Defender exclusion lists? Might be a bit sketchy though 😦