README: document PIN entry depedencies

doc/README-PINENTRY.md

@@ -1,5 +1,17 @@
 # Custom PIN entry
 
+In order to use the default GPG pinentry program, install one of the following Linux packages:
+
+```
+$ apt install pinentry-{curses,gnome3,qt}
+```
+
+or (on macOS):
+
+```
+$ brew install pinentry-mac
+```
+
 By default a standard GPG PIN entry program is used when entering your Trezor PIN, but it's difficult to use if you don't have a numeric keypad or want to use your mouse.
 
 You can specify a custom PIN entry program such as [trezor-gpg-pinentry-tk](https://github.com/rendaw/trezor-gpg-pinentry-tk) (and separately, a passphrase entry program) to match your workflow.