ntrboot: add USPS warning

nds-card is currently shipping using Hong Kong Air Mail; however, USPS
has blocked all shipments from China / Hong Kong. Add a notice to
describe this.

Author: lifehackerhansol

docs/ntrboot.md

@@ -1,5 +1,11 @@
 # ntrboot
 
+::: danger
+
+The United States Postal Service (USPS) has [suspended incoming packages from China and Hong Kong](https://about.usps.com/newsroom/service-alerts/international/suspension-of-inbound-parcels-from-china-and-hong-kong.htm). All product links listed below may not ship to the United States. For further support (in English), join [Nintendo Homebrew on Discord](https://discord.gg/MWxPgEp).
+
+:::
+
 ::: tip
 
 If your flashcart comes pre-flashed with ntrboot (or you have already flashed ntrboot to your flashcart), you can skip to [Installing boot9strap (ntrboot)](installing-boot9strap-(ntrboot)) for instructions on how to use it.